Knitting books, as opposed to magazines, are publications that you'll likely keep for some time. For this reason, and the cost investment you'll make, it's important to choose a knitting book you can use for quite a few projects. If you're only drawn to one or two patterns, it's probably not worth the money you'll buy the book. You'd be better off buying inexpensive, individual project patterns similar to the ones in the book. If there are at least five or six projects you'd like to make in a knitting book, however, then it may be of good value to purchase.
You can increase the value by making the projects in your knitting books more than once; creating the patterns in different stitches or colors changes the result. You can also make clothing in different sizes for friends and family as gifts. A knitting book full of home decor patterns such as place mats, pillows, rugs, potholders, dishcloths can also provide you with an endless variation of gift projects as well as crafts to make for your own living space.
An excellent knitting book to choose is one that has photographs and directions for hundreds of different stitches. This allows you to keep learning new stitches so when you master them you'll be able to use them to remake old projects from your other knitting books. With a book of many different knitting stitches, you could also knit practice squares and perhaps stitch them together to make an afghan. This type of book may be called a stitch glossary or dictionary. It can be fun for all types of knitters to experiment with and practice new techniques, but make sure there are plenty of stitches in your skill level.
If you're a beginning knitter, a book that teaches you to knit may be a good idea, if you'll later be able to remake the simple projects such as scarves and slippers by using different, more complex stitches. You could also reuse beginner patterns when you're a more experienced knitter by experimenting with multiple colors rather than using just one color. It's important to make sure you understand the instructions clearly when buying a beginner's knitting book or you may not be able to complete any of the projects.
If you want to buy a knitting book that has more advanced projects, make sure the patterns are worth the extra work. Advanced knitting projects may take longer to make due to extra details or complicated shaping. For example, some kinds of lace that feature complex stitches and stitch changes on every row take a lot of concentration as well as dedication to complete. If you really love the patterns, then the book is probably right for you.