A few years ago I bought and read the 50th anniversary edition of "The Philadelphian", the great best-selling novel on which the movie is based. The book was published originally in 1956. Robert Vaughn, one of the key actors in the film, wrote the foreword for this edition, and one of the deceased author's daughters wrote a preface.
I've probably read the book 20 times and watched the movie 10 times. As good as the film is, the book is even better. While the book goes into depth of one of the character (Tony Lawrence) of the fourth generation, the book deals with all four generations. There are some differences, but you'll easily recognize the characters. One of the great things is that as I read the book, especially the second half, which dealt with the 4th generation. I could actually picture the characters in the book from how they appeared on the screen. I think you can still buy this new on Amazon or Barnes & Noble, and if not, you can easily pick up a very good used copy from Ebay or Amazon. Especially if you are from Phila. like I am, you will love the book. Not that I cavorted with the upper class main line characters, but I could relate well to them and the other characters and geographical settings as well.
I'm looking at the paperback edition right now- 322 pages. It's just about the right length. If you like the film, most of you will like the book even more. You won't however, get answers as to whether William Lawrence was gay. My own view is that neither the book nor the movie provide other than speculative information on this question.