In addition to the monster rules in the main Heroes & Other World rulebook, this supplement introduces two systems to determine how monsters act: Fight-or-Flight, which is a simple 3D6 test to see if a creature flees or stays in the fight; and the more detailed Reaction chart with responses ranging from continued fighting to a blubbering total surrender. There are also new rules for Spell-like abilities, which consume ST just like casting a spell does.
Although most creatures found in The Tome of Terrors and Treasures are from the Dungeons & Dragons 3.5 System Reference Document, there are a few new creatures scattered about. For example, in addition to the SRD's dinosaurs (deinonychus, elasmosasaurus, megaraptor, triceratops, tyrannosaurus) the Tome includes the allosaurus, ankylosaurus, brachiosaurus, compsognathus, pterodactyl, stegosaurus, and tylosaurus.
Flipping through the pages, one notices a distinct lack of Celestials: There are no stats for Angels, Devas, Eladrin, or Guardinals. I think the reasoning behind this is that "Good" or "Lawful" outsiders shouldn't come up as "monsters", a mindset that apparently comes from the OSR movement. But Brandon also excises more mundane Good/Lawful creatures like Blink Dogs, Coautls, Lammasu, and Metallic Dragons. While I understand the reasoning behind these decisions, I feel that it skews the implied setting in the book; it's almost like some fantasy version of a Warhammer 40,000 Death World where Dwarves, Elves, Halflings, and Humans stand against a never-ending tide of savage humanoids, demons, devils, and undead. Granted, some people may like such a bleak picture but I doubt Brandon intended to paint HOW's setting in such a way.
There are also a few excisions of the more bland entities of the SRD like the Athach, the Krenshar, and the Tojanida. A few creatures seem to be have eliminated to reduce redundancy: Tritons were cut while Merfolk made it; there are Succubi but no Erinyes. I wish Brandon had also tossed one of the "small evil fiends used by wizards" but both Quasits and Imps make it in.
The organization is a bit muddled. There's a clear distinction between the first three sections of monster rules (Animals, Dire Animals, Dinosaurs) and what comes after it ("unnatural" dangers) but I'm not following the train of thought behind this organization. For example, the ten sections following Dinosaurs are: Fauna and Fungi, Oozes & Slimes, Vermin, Swarms, Aquatic Terrors, Beasts, Demons, Devils, Dragons & Wyrms, and then Elementals. If you'd think that a section Fairies & Fey would be between Elementals and Giants, you'd be mistaken. Fairies & Fey are placed after Humans & Demi-Humans and before Golems. In addition, some monster are in unexpected sections, like Azers in Humanoids (with Goblins and Orcs) or Gnomes in Fairies & Fey. Fortunately there is an appendix that lists all creatures in alphabetical order with page numbers, although a few typoes make looking for Imps and Human Mercenaries take a little longer.
There are 365 pages of monsters fit for all sorts of environments and ranging from decent challenge (most animals) to guaranteed TPKs (Balors). While the fact that ToTaT is based on the D&D 3.5 SRD should make other conversion from D&D easy, I've find it's a bit hard to do direct conversions in any case besides playable races.
Speaking of Balors, weighing in with a mighty 265-300 ST and the ability to automatically summon 4d6 Dretches (9-15 ST each) or 1d6 Hezrous (120-140 ST each) for a cost of 5 ST. Even assuming that summoned creatures can't summon others (a rule in D&D 3.5 but that doesn't seem to be in ToTaT), a Balor can very quickly create a small army that can steamroll most parties.
As you can see, the Tome certainly delivers on the Terrors. Soon, I'll look at the Treasures in part 3 of my review (coming SOON). I hope I'm not coming across as too harsh in these reviews.