This is the obligatory list of books that I have found helpful in one way or another. It's not a complete list, and there are many books that you'll find on other Pagan reading lists that are not here. Partially because they are everywhere else - and partially because this isn't intended as a "these are the books you have to read to be Pagan" type list. These are just books that I've read that I've found to be informative, helpful, or inspiring - and many reflect my own varied path and personal biases and preferences... Take it or leave it for what it is.
American Gods by Neil Gaiman: While I'm doing this list alphabetically, I'm glad that I can present this book first. Yes, it's a work of fiction, but I feel that it captures the essences and spirit of the gods and how they can be seen to have adapted to modern life, as it were. The representations and ideas expressed in this book are just excellent, and should be at least considered by anyone studying the philosophy of such ideas.
Bard by Morgan Llywelyn: Another work of fiction, a historical fiction this time, about Amerghin and the Milesian invasion. Yes, it's a piece of fantasy and speculation inspired by history, and I'm not recommending that people read this and then try to present it as history. That said, I feel this book, and others written by her, capture the spirit and essence of the Celts within the scope of the modern myth - some of which could also arguably be called historical fiction.
A Brief History of the Druids by Peter Berresford Ellis: More for those interested in the Celts and ancient Druidism, a good read that pieces out what information we do have from the various mythical versions often presented. It is dry in parts and written in "textbook style", but has good information for those interested in this particular area.
Druids by Morgan Llywelyn: Another work of historical fiction by Ms. Llywelyn, this time from the perspective of the Gauls fighting off the encroaching Romans. Again, probably more fantasy than fact because of its speculative nature, but again captures the essence of what it may have been like beyond the dry annuls of academia and the completely New Age concept of hippy like demi-gods. **Yes, I have a particular fascination with the Druids, from both a historic and a romantic perspective... and while I acknowledge that this book is more romantic than historic, it breathes life into figures which are too often presented as two-dimensional caricatures. In other words, if nothing else, it helps us remember that there were real people back then - something I feel we often forget.
History of Magic and the Occult by Kurt Seligman: A more academic book (that is, a bit dry at times), the title pretty much says what it is. Starting with Sumero-Babylon and working its way up, it's pretty comprehensive in the history (i.e. it's not a "how-to" book).
The Myth of Matriarchal Prehistory: Why An Invented Past Will Not Give Women a Future by Cynthia Eller: Admittedly, neither Ms. Eller nor myself seem to be entirely unbiased regarding this topic, and it's been a thorn in my side for some time. The theory of the prehistoric matriarchy where women ruled, where Goddess was central (if a god is even mentioned), and where everything was peaceful and perfect because men understood our power because we can bear children has been debunked by most reputably anthropologists I've read or read about, and yet the myth persists. Cynthia Eller not only attacks the myth itself, showing how it seems based more in wishful thinking than in the actual archeology, but she attacks the myth itself and argues that it's really not as empowering as women think. I won't belabor the issue, as I've already done that elsewhere, but I will say everyone on either side of this debate should at least give this book a read.
The Principia Discordia: It may not be for everyone, but I still think everyone should read it. If nothing comes out of it other than maybe we learn to laugh at ourselves a little bit more, then that's okay too.
Small Gods by Terry Pratchett: This isn't the only book in the Discworld series that I would recommend for its philosophy, but it's the first one that always springs to mind. In his series, Terry Pratchett has given us some great metaphysical ideas and theories, which, I feel, often hit the mark more often than his non-fiction counterparts.
Tao Te Ching by Lao Tzu (as translated by Peter Merel): While not strictly "Pagan", I found this book, some passages in particular, to be very inspiring and it helped me embrace paradox and balance and other key issues I think often overlooked.
The Way of the Shaman by Michael Harner: A good introduction to Core Shamanism and shamanistic techniques. I don't agree with everything in this book, but it's still a good book for core techniques for those interested. The critiques of this book that it's antiseptic and devoid of cultural symbolism are true - as Harner specifically tried to find the related core elements. In my opinion you can easily build on this with your own cultural understandings and symbolisms, as I feel is ultimately the point of the neoshamanic movement.
The Way of Wyrd by Brian Bates: Well, aren't you lucky? It's back in print - and I had to shell out the cash when it was rare and hard to find. This book is written as a sort of historical fiction, which I feel captures the spirit of what it presents better than anything else could. Wyrd is one of the best presentations of a 'fate' concept that I have ever found, and this book highly influenced my own conceptions of it.
Wicca: A Guide for the Solitary Practitioner by Scott Cunningham: Yes, I did just belie what I'd said above, and yes, this is on many Wicca 101 lists. I'll also agree that this is a watered-down version as compared to the more traditional Wicca. That said, it was the first book I'd read, and so holds a special place for me. Also, I do think it can be helpful for beginners, and I don't really have a problem with the watered down variety of Wicca, as long as it's recognized as being divergent from the original.
Some are fun, some are informative, some I just think are interesting - and most can be found elsewhere on the site...
Ár nDraíocht Féin: A Druid Fellowship: The home page of ADF - a neoPagan Druidic movement.
Asatru Folk Assembly FAQ: Some information of Asatru
Axioms and Assumptions of a Magical World View: Based on the idea that reality is how we perceive it, discusses the assumptions which are applicable to a magical reality.
Creation Myths from Around the World
Creation Myths, Flood Myths, and Afterlife Myths from Around the World
Defining Paganism: Paleo-, Meso-, and Neo-: This article by Isaac Bonewits does what it says it does.
Encyclopedia Mythica: Offers write-ups of several cultures mythologies, with individual entries for the various gods and goddesses. A useful reference, though I recommend using it as one of many, for there are things written in several of the articles that I found to be less than accurate.
Exploration of Dark Paganism: Geared particularly towards people like me who are tired of the insistence on "light and love" in some neoPagan circles. I don't agree with everything on the site, but most of it is quite good.
Emerald Tablet of Hermes: Has some translations of the tablet from whence "as above, so below" comes from, referring to the interrelation of the macro- and microcosms.
Fluffy Behavior 101: An essay about what qualifies as fluffy behavior.
"Fluffy Bunny?": Offers links to other fluffy bunny guides, as well as articles on why Silver Ravenwolf is hated.
>Golden Dawn Glossary: A list of meanings of words often used within the Pagan and magical communities.
IMBAS Homepage: A site dedicated to Celtic Spirituality.
Law and Limits of Hermetic Magic: While I'm not a practitioner of hermetic magic, I found these concepts interesting and applicable to others as well.
Pagan Authors to Avoid: Well, the title pretty much sums it up. You'll see some authors on this page that many others recommend - but I agree with the author. Actually, I'd add several other names that aren't mentioned on the page, such as Edain McCoy, Barbara G. Walker, Fiona Horne, and various others.
An Overview of Core Shamanism: Is what is says it is.
Recent Developments in the Study of The Great European Witch Hunt: This article reviews the latest evidence and information regarding the witch trials, and dispells many of the previously held, and oft repeated, misinformation regarding them.
Religious Symbols Dictionary: A visual glossary of religious symbols from various paths, not just Pagan.
Sorting Out the Pagan Traditions: Brief overviews of and links to information about some of the various traditions and paths which fall under the Pagan umbrella.
When is a Celt not a Celt? An article by Joanna Hautin-Mayer which examines some popular books which are meant to be historically accurate Celtic traditions, which aren't... as well as reviewing some books which are.
Wicca for the Rest of Us: Presented as a Wiccan Countermovement in an attempt to reclaim Wicca from the "fluffy bunnies". Has some very good information on "Real Wicca" and also some issues such as the Prehistoric Matriarchy theory, the Burning Times, persecution complexes, etc.
Wiccan Ethics and the Wiccan Rede: Offers different interpretations of the rede, specifically the difference between the "modern reconstruction Rede" which becomes a prohibitive commandment, as opposed to the "actual construction Rede" which is seen as permissive advice.
Why Wicca is Not Celtic Paganism: An article which discusses some of the differences between the practices and beliefs of Wicca and those of Celtic Paganism.
Why Wiccans Suck: A rather scathing condemnation of the fluffy bunny variety of Wicca. Was written as a rant and so is sarcastic and over-bearing at times, but still contains some poignant truths. (The site itseld seems to be gone, but WiccaWeb.com has hosted several of its pages.)
(Updated on 7/29/09 with corrected links and an author's note to self to add more links.)