It's possible that many superfans of the series know a lot of the information contained in here, as the book does draw on modern sources like the 50th anniversary collection. However, the authors also conduct some new interviews with the likes of series producer Arthur Marks (who is a big source of info on the aforementioned DVD set), and besides, the organization and presentation of the material is superb. For a relative newcomer to the series like me, it's an essential companion as I go through the episodes.
There is a fine general summary of the character's literary origins and the development of the TV series before we get into the heart of the book: A comprehensive episode guide with production info like airdates, cast lists, and plot summaries (though the authors are scrupulous about not revealing the identity of the guilty) along with notable quotes and trivia. Each season begins with an overview that has valuable info about the direction the series took that year, and many episode entries feature sidebars and information that isn't necessarily only relevant to that particular installment, but is often compelling general material. Therefore to get all the good nuggets about the show, you have to read all of the episode entries, but you can get a real understanding of Perry Mason by reading the beginning and the season overviews, then saving the episode guide entries for when you get to those points in the series.
The hefty book is also filled with amusing appendices, such as "times the killer was revealed outside the courtroom" and notable performers who could have but never were on the series. There is brief coverage of The New Perry Mason, but in-depth treatment of the 1980s TV movies with Burr reprising his role.
In short, the book is not in short; it's over 650 pages of useful info. The writing is professional, as you would expect from a talented veteran like Robertson, but it is playful when appropriate and celebrates the fun and enjoyment of experiencing a classic vintage TV series. The authors appreciate the series and know they are going to be read by diehards, but they also make sure to be accessible to newcomers. The Case of the Alliterative Attorney gets my highest recommendation for television lovers, and I think even "expert witnesses" well versed in Perry Mason will enjoy having all this stuff in one well-organized volume.