Some other guidebooks tend to go a bit mad with the colour photographs, and skimp on the information. The Rough Guide majors on information, and has just a couple of small sections of photographs.
For me this is better, and certainly more useful if you want to know which bus goes to Hafnafjörður, for instance. If you want to look at pretty pictures there are other guides.
As always the Rough Guide is the one to get if you're touring around. The various regions of Iceland are described in some detail, with good advice about where to stay if you are driving around.
I've been to Iceland a couple of times. The information in this book seems to tie up pretty well with what I know, so I would be inclined to trust it, on the whole. I wasn't particularly impressed with the pronunciation guide though which seemed to have a couple of errors in it.
The book is well laid out, attractive, easy to follow, has reasonable maps, though not in colour, and is packed with relevant information.
But if you're planning to stay in or around Reykjavik on your trip, most of the book will be of no practical use to you though it may be of interest. If that's the case, I would recommend the excellent pocket-sized Footprint Guide to Reykjavik instead.