I have/had books 1-3 from the Mongoose imprint. I also have most of the books from the multiplayer
RPG (sounds weird describing a table top
RPG that way) Mongoose published -- but they are not
d20-based
AFAIK. As I recall, they are based on the same system as used in the single-player books.
Here's my review of Book 1 of Mongoose's run:
http://www.dakkadakka.com/dakkaforum/posts/list/341881.page I hope they do a reprint of the Companion. Looked to be a great book.