Basically, the book deals with category fiction, and then in each category, discusses the basic plot-types, plot pitfalls and things that are absolutely necessary to include if you want your book to sell. In fact, compared to Manual for Fiction Writers, Writing Popular Fiction is mercenary in terms of how it assumes that the whole point of writing (or being an author) is to sell something.
The category fiction discussed in the book is: Science Fiction and Fantasy, Suspense, Mysteries, Gothic-Romance, Westerns and Erotica. If your future novel is going to be in any of the above categories, you should definitely take a look at this book. But anyway, there are quite a few chapters on writing in general, which make it worth looking at even if you don't think you are writing in any of these categories.
My biggest takeaway from this book was that it made me realise that my NaNoWriMo novel is going to be very close to (if not actually) Science-Fiction. I don't have any new technologies or such, but I'm definitely writing a "In The Near Future" story. And from that section alone, I have a lot more things to consider about the background of the world I'm creating. I may not include all the information, but I have to consider them if I want my novel to feel realistic.
Towards the end, some of the advice gets very dated, especially when he talks about submitting your manuscript (and typewriters). In the age of e-publishing, you can completely by-pass the traditional gatekeepers if you're so desperate to get your book out. Even if you want to be a traditional published author, I have a feeling that the submissions method/getting an agent process if different. But then again, I don't work in the publishing industry so I don't know.
If you're stuck on plot, this is definitely the book to read!