They are often the authors first book, heavy on the white space, with large fonts and so on, haphazardly organized, and night and day from something like a Pragmatic Programmers title.

I'm as susceptible as anyone to the copywriting strategy of "this book will solve your particular problem and, if you save even an hour, it pays for itself" but too many launches of this type have soured me on self published business and technical books unless there is absolutely no other place to get the information.

As a rule of thumb, if someone is selling a book in multiple "packages" , to me its an indicator that the quality will be poor and I will regret the time wasted reading it.