This is a topic near and dear to my heart. I am a birder and involved in wildlife rehab in the US, so wildlife guides have been in my life big time for years. I checked out as many wildlife-related books from the public library as I could before deciding on which to buy. What I like about wildlife books is lots of pictures, a bit of text, and maps or descriptions of where the critter is likely to be found--all on 2 opposite-facing pages, if possible. Bonus points for a small-enough size to fit in a pocket or easily slip in and out of a daypack.
I settled quickly on Fitter, Fitter, & Hosking (2000), published by Princeton Pocket Guides. There is a new edition (2007) that has the same size and exactly the same # of pages--but costs almost double (at least at Amazon). You can see the new version on Amazon here: amazon.com/Wildlife-Galapagos-Travellers-Dan… If you click on "paperback" under the "formats" box, you get to the older version. (I'm not pushing Amazon--just an easy way to see both versions.) I haven't looked at the 2007 edition, so I can't speak about it. But the 2000 edition is terrific and was all I needed. I've used it extensively since I came back too--it's now dog-eared and covered with invaluable notes. Perhaps someone on the forum has looked at the 2007 edition, but it's hard to believe it could be worth twice as much when it has exactly the same # of pages.
These books are both compact in size. slipping easily into most of my shorts pockets or the daypack. But the reality is you don't need a field guide at all when you're on the landings. Except for the various "Darwin" finches, the IDs are pretty easy. The mammals are distinctive; there are only 60 or so species of birds--13 of which are those pesky finches. And your naturalist guide will have identified and pointed out the identifying characteristics long before you could look them up in your book. I found that I used my field guide mostly before we traveled; in the evenings during the cruise when I was going over my notes; and back at home, writing up our trip report. So portability and size may not need to play as much a factor as I originally had thought it would.
The only caveat I offer about this book is that it has nothing about fish. That didn't matter at all to me. But if fish are important, this might not be the book for you. Our boat had several terrific field guides in its library that were dedicated to fish, which our group used extensively. Of course, it didn't hurt that we had a retired marine biologist in our group. ;-)
Well, THAT's probably more opinion than you wanted or needed! Having written all that, a field guide can be a very personal choice. And all of this is just one person's opinion, of course. Some people like lots of text; others, as little as possible. Some photo arrangements can make you crazy; others fit right into your reading/learning style. I suggest you get several from your public library, if you have access to one, and read them through to see which fits your style. I couldn't find any in our local book stores, so that may not be a useful option.
trip report at http://galapagos2009.wordpress.com/