There probably is not one book that will answer all the questions you might want to ask. I can, however, suggest a couple of books that may be helpful if you can describe what type of turning you are currently trying to improve e.g. bowls, spindles boxes etc.
Thanks Dalboy for noting my free publications.
I had the intention to produce a whole series of free publications for turners but received little feedback on what might be helpful so stopped a couple of years back. Maybe that's the problem of 'free' there is no way of judging what other people conceive as helpful unless they say. By that I mean there are no sales records to be able to judge popularity. For instance I started producing a publication on profiles and a publication on practical design methods for beginners but stopped. I don't think people who use free resources understand the amount of work involved.
Design for woodturning seems to come low on many turners' lists of priorities and for this reason publishers will always concentrate on the 'how to' with tools an equipment where sales can be more or less guaranteed. They pay the writer and production costs and have to make a profit so are mostly targeting beginning turners or those interested in following a particular technique, method or style of work produced by established or popular turners.
I am aware I am probably expressing a jaundiced view from an isolated standpoint.
Philip
Bob, I always seem to create problems when I contribute on Forums so if you either email or PM I will gladly help you further.