Not having any significant knowledge of butterflies, it is difficult for me to know how reliable Butterflies Worth Knowing is. First of all, much has changed since it was copyrighted in 1917 and published in 1925. Large swaths of butterfly habitat have been destroyed, migration routes are different, and climate conditions are changing. Nonetheless, much in the work is in all likelihood quite reliable.
The introduction to Butterflies Worth Knowing describes the behaviors, life histories, and attributes common to most all butterflies and provides a little advice on photographing and collecting butterflies. It then provides entries for scores of butterfly families, tribes, and individual species found in North America. Accounts of the discernable features of the butterflies are likely still reliable. The descriptions of their life histories, though, may be less so. The author, Clarence Weed, frequently acknowledges that there are gaps in the current understanding of one or another species. Indeed, he ocassionally suggests a fertile research topic for young entomologists. His very caution makes one wonder if there also might not be mistakes in his settled understanding, particularly as the growth of our knowledge of the natural world has changed significantly since Butterflies Worth Knowing was written.
Regardless of these doubts, reading the work cover to cover (as opposed to using it as a reference book) likely leaves the lay reader with a better impression of North American butterflies generally. Unfortunately, reading it cover to cover subjects the reader to a repetition of details describing slightly varying species, making it a bit mind numbing. The best use of Butterflies Worth Knowing would be to consult it in conjuction with a recently published reference work on butterflies to see just how our understanding has changed.