Well,
DR td, DR JoseSPiano is much more qualified to do this than I; I just don't have time to read the book right now. But at a quick skim:
It's your standard coffee table book,
a la the Wicked Grimmerie, or some of the other shows that have a "book" out now. Nice presentation encasing three different tomes:
Mary Poppins Design Folio, which contains creative team sketches of the costumes and set designs, along with selected quotes from the team. Nice feast for the eyes, but pretty brief.
Mary Poppins On Broadway, which could be the special souvenir book sold at the theatre, except that it probably contains a lot more photographs. All stage shots, with detailed captions.
"If you want to find Cherry Tree Lane...", which contains the meat, looks to have a good balance of narrative to photos. It covers, briefly, the original books and the movie, then spending the bulk of concentration on the adaptation to the stage musical. Shots of the creative team at work, the actors, etc., etc.
Is it worth the $50 list price? [I obtained it for substantially less.] Well, that would be up the collector. I'd say if you're really invested in the show, it would be worth it.