The Gentium fonts are that big because they contain not only Roman (and accented Roman) characters, but also Greek and Cyrillic and IPA and maybe a few more sets, all designed to look nice together in the same typeface for multilingual usage.
The full Gentium font might possibly be overkill for your book, and I think there's a "light" version that only has the Roman+accented characters if you're worried about the space they take up.
ETA: Here it is:
Gentium Basic. Just the accented Latin characters and a few others.