It's an incredibly useful book, not only for the natural history/ecology info it gives, but also for the references it provides. If you want to know about American sals but can't look up the actual scientific papers, then this book is the best there is, period.
The "errors" in it that I hear about most often is the taxonomy, but that is no mistake. Petranka just has differing views about species concepts than some of the other field guide authors (notably Collins). There are some other range map errors but he identifies those in the introduction.
One real error that I found was that the photo of the larval Eurycea multiplicata is definitely not E. multiplicata! But that's it...
