Chapter 9 discusses potential holes in the title list according to the reader's age group; there may be books on a given subject for beginning and middle-school readers, but nothing geared toward young adults. This is one kind of void to look for when doing your research. Other voids may be found in general categories (although it's fairly rare to come up with a completely new topic) and in individual publishers' lists.

Take this book as an example. Adams Media, the publisher, has a book called The Everything® Get Published Book. But there was no title on Adams's list that focused specifically on writing book proposals. This book filled a void in Adams's Everything® series. The slant is different enough to distinguish it from the other, related book, and adding this title gives Adams more comprehensive coverage on subjects likely to appeal to aspiring writers.

That really is the key to filling a void, whatever that void may be. Combine a clear idea of your reader with a topic carefully structured to match that reader's needs, then look for publishers who have already demonstrated, through their title lists, that they are interested in the same reader you want to reach. If you can do that, you're already miles ahead of many writers.