Hal Koerner is one of the most recognizable american ultra runner. Double Western States 100 winner(2007, 2009), Hardrock 100(2012). Out of his wins, he is an extremely positive and always happy person.
Field Guide to Ultra Running is kind of compendium for runners. Definately for trail runners as for road athletes.
Koerner describes ultra trail running in details. Some parts of this book, for more experienced runners can sound little bit trivially. From the other side, the chapter about training is mostly about training for professional runners. Sometimes it can sound as if the author could not decide who is the reader of this book. This is, in my opinion the only weak part of this piece. I would totally recommend it. You cannot find on the market anything better about trail and ultra running in a handbook form.