(This review is based upon my own copy bought in the early 1960's, since which time the book may well have been updated, but it has never been out of print. That fact alone says a great deal).
A previous reviewer is right to say that this book lacks TAB and requires readers to learn to read music, but therein lies one of its enduring strengths. Bert's book may be old fashioned by modern standards, but it progresses slowly and smoothly through all the rudimentary stages of learning to read music and to associate notation with positions on the fretboard.
The book is also unusual in that it is specifically designed for would-be guitarists who are teaching themselves to play plectrum style guitar. The fact that so many of today's most influential guitarists credit Bert and this book with having inspired and guided them is testament alone to the longevity and popularity of both in the eye of both public and modern musicians. Although rhythm playing is featured (and remember that in his early days, Bert was both a soloist and rhythm guitarist in several big bands), the main focus of the book is upon plectrum-based melody playing.
'Play in a day' doesn't spoon-feed the reader. It assumes both intelligence and a willingness to persevere but is nonetheless clear, written in mostly jargon-free language, isn't condescending and it lacks all traces of flashiness and trendiness. In short - it's a classic, which is why it has remained on the bookshelves and has retained its standing as a bestseller for over 40 years.
Also worth trying to find is the rarely-mentioned follow-up book that Bert wrote called 'Play every day', which is just as good and which covers some of the more advanced stages and styles of solo guitar playing.