"How to draw manga" books are not that useful IMO. They can give you some useful tips, but they can't teach you how to draw. If you only want to get them to improve a bit, it's ok, but don't expect them to be very useful. As any other "how to" books, they can only give you very general directions. Some people who buy them to learn how to draw will end copying what's in the book but then having to draw anything by themselves, they won't be able to do it. To draw you need exercise and patience to look at others' works, which is something I guess you do very often. Books are only a way to know how to make the process a little less complicated since as I said they can give you tips. But you need to do all the work by yourself or you'll stick only to what the book says and won't improve at all. Try to go to the bookstore and ask if you can look at the pages to see if the book is really what you need. Sometimes you get something, then go home, open it and discover it's not what you wanted. If you don't know how these books work, try asking to take a look at it
