Fiend Folio

Personally, I found this sourcebook to be rather unpolished and lacking in focus or theme. It just feels like a random collection of monsters with a tendency towards to weird, unrealistic, and the downright ridiculous (examples: a human-torso devil with a lower body of a snail who lairs in a river, and an “all-legs” spider aberration, and a fat manipulative devil who likes to put on make-up).

There are a few gems, like the sarkrith and the kaorti - new species that you can build a campaign around. But in general, these are few and far between, with many feeling closer to being rehashes or variants just for the sake of it. There's nothing wrong with just taking these as ideas for further improvement, but the impression I got was that many of these monsters could do with a few more design iterations. For example, there's this gargantuan worm who needs to swallow humanoids so that its larvae can burrow inside in order to feed and grow - the problem: it takes 100 years to grow up to gargantuan size... in the same corpse. Another oddity (to me) is that there's quite a few aberrations and plants and magical beasts that cannot speak, yet understand languages, despite having a description that are at odds with how or why it could understand languages. I get that it's “magic”... but still...

As for the “extras”, there are a few new templates scattered throughout the book, as part of the entry showcasing the template. Several entries introduces new materials, equipment, and items. Finally, the sourcebook also features three prestige classes for fiends (not too bad), and a small section on grafts and symbionts (which can also be found in other sourcebooks).