Having visited the Galápagos and planning my return, I can say this introductory booklet (140 pages) is a reliable for people who want an overall idea of the ecology and want to identify some of the most prominent wildlife they are likely to encounter - it is easy to understand, has good information, decent photos of the main players and illustations of the commonest landing sites. For you, this booklet rates three stars for clarity, comprehension level and lightness in your backpack, but it fails next to Andy Swash and Rob Still's more comprehensive and better illustrated "Birds, Mammals and Reptiles of the Galápagos Island".
For those who want a more in depth knowledge of the Galápagos and their wildlife, or are traveling to some of the more remote sites or the northernmost islands: you will feel short-changed and wish you had purchased more comprehensive guide(s), so you can identify all the critters you will possibly observe and learn much more about their habitats and behaviors, climate and cvlimate zones, etc. For those travelers (birders, nature buffs, divers, etc.), this booklet rates a generous three stars for its relatively topical treatments.
Choose your guidebooks according to your interests and skills. And go there, walk softly and leave nothing. Visiting should have as little impact as possible, and in my opinion when done properly does help protect these wonderful islands of biological diversity from rampant pirate fishing, oil spills and short-sighted political stewardship. Our "tangibilitized" interest is the only one the Ecuadorian government- at least up until now- has responded to.