There are as many quilting books published as there are unique quilts created. Some focus on quilting from an historical point of view, some supply patterns or introduce techniques, and others serve as reference material.
Finding books worth adding to your quilting library is a matter of matching your priorities with what the book offers. A "how-to" reference guide, for example, better serves a novice quilter, as opposed to a lengthy diatribe on improving appliqué technique.
In addition to the type of book, the skill level for which it is written is equally important. For those seeking to learn a skill, general, introduction-style books are the best choice, while more detailed works better serve experienced quilters seeking to refine their techniques.
These book reviews can help you find a quality resource :
Classic Four-Block Appliqué Quilts
Egg Money Quilts
Encyclopedia of Pieced Quilt Patterns
Flawless Hand Quilting
Quick Quilts to Make in a Weekend
Pieced to Fit
Still Stripping After 25 Years
The Modern Quilt Workshop
The difference between a helpful and a mediocre quilting book often has a great deal to do with the author. Quilters, quilting instructors or quilt historians are most knowledgeable on the subject. Scan the interior pages for information on the author's experience when considering the value of any quilting book.
Also worth consideration is a book's depth. How extensively a book covers your topic of interest effects how well it rounds out your collection. If you are anxious to increase your collection of patterns, for example, choosing between a book of five patterns and a book of twenty-five patterns is fairly easy.
Usability should never be overlooked when you compare books. How efficiently a book is organized, and how clearly information is conveyed effects how often you are likely to actually use it. If you process information presented graphically better than text, for example, a book making extensive use of photographs will probably make a better teaching guide.