This article describes the source books you'll need for this campaign. Some are required to use the material as is. Some are simply recommended reading.

I've used a mixture of GURPS and 2nd edition AD&D books as background information for this setting.

PDF versions of these books are available on the web for a very low price. The scans are rather low quality, but all the text is readable; they've got an occasional tendency to be slightly rotated and in one case, the pages were out of order (the Planes of Law PDF from RPGNow.com).

Fold-out maps are the most butchered part of them, with each being cut into 8.5×11 pages for the PDF and missing small amounts of material along each edge. Map intensive products suffer the most, such as boxed sets.

Speaking of boxed sets, they jam the entire set into a single PDF. Ugh. It took quite a lot of effort to cut the files into properly sized individual PDF files for each book in the sets. It was worth it though, as they're much easier to read that way.

Most of these issues are just slightly irritating, but the books are still usable. Pay attention to the user reviews on RPGNow.com. They'll tell you if there's any totally ridiculous problems with the PDF, before you buy it. For example, I don't know if it was ever fixed, since I didn't buy it, but The Night Below (not used in this setting) has very bad map scans, so bad in fact, that on the page you can purchase it from, the warning about this is in bold. This was actually made apparent to the RPGNow.com folks through the reviews, so they put up their own warning about it. I guess they aren't the ones that did the scanning, though, so they can't fix it.

I bought all of my collection of AD&D PDFs from RPGNow.com, but there's another site, paizo.com that also has them. I know nothing of the scan quality on paizo.com. The reason I avoided their books is that they are water-marked with publisher information (gets in the way of seeing my books the way they were intended; as if the friggin' publisher information isn't already in the book!). Most of the books are between $4 and $5. Paizo.com has lower prices, but water-marking, which annoys me.

If you're going to go this route to get these books, I recommend finding and learning how to use some kind of PDF cut and join tool so you can fix the boxed sets into separate files for each book in the set. Do not pay for such a tool; there are free alternatives out there that work much better. I used a command-line tool available in the Ubuntu Linux repositories for this. It's called 'pdftk' by the Synaptic Package Manager.

GURPS Books (with links to their product descriptions):

• Dungeon Fantasy 1, 2, 3, 4, 5, 6, 7, 8 - This PDF-only line of books is awesome. It captures the old AD&D feel in GURPS quite well, at least if you're into hack and slash, experience points, barbarian warriors, etc. It also has a few unique PC races and the fifth one has detailed rules for familiars and allies. That one is worth every penny, but even twice every penny would have been worth it. You could muddle through without these, if you're strapped for cash.
• Egypt - This is required for the island of Egypt, though most GMs could probably fake it long enough for a quick visit by doing a quick web search. If you're going to do more than a quick visit, you should either do a lot of research or just buy the GURPS book.
• Martial Arts - I am using this book extensively, so you're going to need it.
• Power-Ups 1 - Another PDF-only book. Gives interesting options for cinematic combat abilities. I used this to build one character, Shoot it With Arrows, of Adventuring Party. It's an inexpensive book, but if you're strapped for cash, just leave the guy out. You could claim Shoot it With Arrows died in battle the week before the campaign began, should it become important.
• Powers - You're seriously going to need this book. The magic system and spells in this campaign make heavy use of it.
• Psionic Powers - I'm starting to make use of this in the spells, as it has a fair number of new modifiers that cover some of the corner cases.
• Thaumatology - Not required unless you're going to use the witches of this setting. They use the Threshold-Limited Magery rules from this book. Includes new modifiers for Magery, some of which are appropriate for use with the magic rules used with this setting. The rules for external energy sources might also be handy.

AD&D Books (with links to their RPGNow.com product pages):

• College of Wizardry - This book is used as background material for the school the Wizard's Guild is based around. I own the PDF and recently got the real version back in my hands. This has one folded up map, so beware of bad scans of it. The folds of the paper go right through important parts of the map, so the PDF version is missing important map details on those edges. Most GMs likely won't run actual adventures in those maps, instead using the College as a source of background flavor for wizards. If that's the case, then the map isn't too critical. Then again, I'm not you, so what I've said may not apply.
• Den of Thieves - This provides useful background information on thieves guilds and provides a fully-fleshed out one for you to use. It's good background information for understanding all the things a guild would have their fingers into, but isn't required. I own a physical copy of this and it has a folded poster, so beware the PDF treatment of it. I may end up adding material related to this, as it has connections to the College of Wizardry product. If I do, it will be as a member organization of the Shadowed Eyes and their role will be minor.
• Planescape - This boxed set contains tiny snippets about the Modrons. If you're just wanting to flesh them out, skip this and go for Planes of Law. This has a bunch of folded maps/posters, which I laminated and hung on the wall; they're just that cool to look at. I only own the boxed set, not the PDF. Personally, I'd recommend finding a physical copy of this one and enjoying the posters every day.
• Planes of Law - This contains information about the lawful planes of the AD&D cosmos, including Mechanus, the home plane of the Modrons. I highly recommend buying this if you want more information on the buggers. Beware of out-of order pages in the PDF, in the portion of the file for the book on Mechanus. It's all in a span of about 5-6 pages around page 12. This is fixable with a PDF cut-and-join tool.
• Van Richten's Monster Hunter's Compendium - I'm using the enclosed guide to witches as background material for the witches of this campaign setting. However, you don't need a copy of this. Everything you'll need (game mechanics) is here. If you want extra detail, though, this is a good read. I've made one major change to the witches in the Archipelago: when they were pulled into the demi-plane, they were cut off from the Weave, so they adapted and learned to channel energy from other planes of existence, with each clan using a different plane.