The biggest problem for me in this is to know how much more/less fabric to purchase. I do make smaller quilts and it usually isn't a problem just to make fewer blocks but the fabric calculation is a problem.
These days, I usually try to get the book from the library before purchasing. If that isn't possible, I will also go to Amazon and see if there is a "search inside" feature to get an idea of how the book is done.
A third possibility is to ask the reviewers on Amazon as people will answer questions.
And, of course, there is the QB. If you ask about a particular book that members have, I'm sure you would get answers.