Good Question!!!
I have a shelf of favorites! Here's a couple:
1. For first time quilters, who are learning on their own: Carol Doak's "My First Quilting Book". It's full of good info, from how to do things, explaining all the little tricks and things that aren't in other books. Plus the book is so humourous, it's simply fun to read.
2. To learn paper piecing - Any of Carol Doak's books on paperpiecing. She explains things really really well, and has a few tricks she passes on. Again, humorous. It's not intimidatin when you read it and do it. Carol is one of the top experts in paper piecing in the land.
3. Marsha McClosky books. She has a couple. I like them for the quilts in them, the instructions and the easy to follow guidelines in making different sized quilts from the same patterns. Also has a great 'basics' section.
4. Again, for new beginners and/or those who need 'results' fast or they get discouraged: try any of Eleanor Burns "Quilt In A Day" books. Clear explanations, great sketches and photos. And will teach you exact 1/4" seaming, how to lock seams, etc. Get quilts pieced fast. Have something to show off fast. Once you master them, it's easy to 'move on' to more challenging piecework, paperpiecing or applique.