Otranto's Arrivals Macabre is a Faerunian book one should keep an eye out for. Unlike many spooky tomes of forgotten and dangerous knowledge, it is not particularly ancient (less than 200 years old) and is fairly common -- a Halruaan printer produced a print run of more than 4000 copies some 100 years ago, and many are still extant, in the collections of various libraries and wizards. It is written in Common.
It's not hard to find information on the book. It is, essentially, a clearinghouse of the author's knowledge of the planes, as well as his suspicions, and some outright rumors. It also contains some spells -- notably all the Summon Monster spells, as well as Planar Binding Lesser, Planar Binding, Planar Binding Greater, Legend Lore, Banishment, Trap The Soul, Symbol Of Insanity, and several variant Protection spells.
Arrivals Macabre discusses other books as sources of knowledge about the planes: in particular, the Necronomicon, De Vermiis Mysteriis, and the Pnakotic Manuscripts are discussed, as are The King In Yellow, the Carcosa Codex, the Demonomicon of Iggwilv, and Xygag's Bestiary. Others are mentioned as well.
Much of the book is devoted to examination of the planes of alignment. In particular, the planes of Baator, the Nine Hells, the Abyss, Limbo, Mechanus, and Carceri are examined and discussed. However, towards the end of the volume, the author gives much thought to a plane (or planes, or demiplane, or whatever it is) beyond the others: The Far Realm.
The author describes it as a place that induces madness in normal beings, a place where the rules of reality are twisted ninety degrees and then rotated a bit. It discusses the genesis of the Kaorti, a race of beings that began as human until their contamination (by attempting to physically visit the Far Realm) and transformation into a posthuman horror.
It also discusses known Major Powers of the Far Realms, including The Eater (a sort of god-mountain that walks, whose only function is to consume; its immense, pillar-like legs end in gaping fanged maws that consume all it walks upon), and other creatures even more disturbing.
Some people have become... "less than sane" for having read Arrivals Macabre, but the consensus is that it's safe enough; many wizards report having read the thing, and of retaining their faculties. In game terms, it can provide a +6 to any Knowledge: Planes check, and a +10 to any check involving the Far Realm. However, it is not possible to memorize the text; even individuals with eideitic memories have tried... and failed. In order to obtain the bonus, one must have read the text first (a task requiring thirty man-hours) and may then spend a half hour perusing the text in order to gain the bonus at any given time thereafter.
Interestingly enough, there are numerous diagrams of protective sigils, runes, and so forth...
...................................................but no illustrations.