When you teach drawing you are constantly trying to find new ways to reach people and explain the art of drawing. I have always had an interest in how to books and usually find them fairly lame and usually are not as good as you think they are going to be (I go by tip number 1 always).
Here are some of my personal tips when looking at 'how to draw' books
1-Look at the Art, does it look good to you? Not just technically good or commercially good, but inspirationally good to you. If the Art does not grab you then what will this person have to teach you? (I do not think this applies to real life teachers though)
2-Do the pictures in the book truely show how to cartoon or are they just excuses to showcase the artists work
3-Are there more examples of Art than just the Authors, does it compare different approaches or art styles. (They mostly do not, and I find that limiting)
4-Can you get the book for free, say from a friend or library or online? The book may not be worth buying, but all books usually have one gem or one good idea in between the covers
5- How to books can only really be starting points or good inspiration, the real learning comes from you actually drawing, which means thousands of drawings before you can see any real progress
6- Take note of terms and tools, compare to other books
What are some good ones that you have come across? One of my favourites and is the very recent Matt Madden and Jessica Abel book, 'Drawing words and writing pictures'.