Most of the books I've seen on CAD tend to refer to various versions of
AutoCAD, which (as I use TurboCAD V9) haven't been that useful. However
an hour or two poring over the CAD book selection in the Public Library
was interesting (and better still) cheap (e.g. free)!

I have a copy of "CAD for Model Engineers" by D.A.G.Brown which has
some useful concepts/ideas in it, and it's not expensive (No 29 in
Workshop Practice Series). He uses something called 'Generic CADD'
which seems to be ideal if you run an older DOS PC, but I suspect most
of us have something a bit better these days. However the ideas are
perfectly useable on any system.

Between that and the TurboCAD V9 Reference Manual (yes - a real book
that came in the box!) I can usually get the results I need - but I'm
still very slow and often resort to pen and paper for simpler things.

Good Luck

Ian