I think that a thematic approach is probably preferable to a big-bucket kind of book. So one that narrows down on "just beasts" or "just intelligent races" is more manageable than one which tries to do everything.
With my proposed
Book of Uncommon Beings (which I sent you a preview of a while back), the theme is unusual beings which are not regularly encountered, but rather which can form the basis of a unique encounter. I'm a biologist myself, so it's broken down into the following categories:
- Variants of Intelligent Races (Reptile Men, Gargoyles, Octopi)
- Titan Slimes
- Animals
- Plants & Fungi
- Otherworldly Beings
There are five entries in each category, which keeps the book to a reasonable size. If you focus on just one category (intelligent races, animals, or plants), then you can be much more expansive within that category. So my vote is for a focused book rather than a catch-all one.