Stephen, off hand I can't think of any specific book I've had or read on retouching, maybe Otto Croy, The complete art of printing and enlarging. i think that hs a section, he also wrote a Retouching book but I've not seen that. There was a small Focal Press Guide as well, just called Retouching.
Most of what I learnt was self taught or from magazine articles. Manufacturers used to sell Retouching kits, I have an old Johnsons one, still using the some of it, it maybe 40+ years old, came with a load of darkroom equipment.
I have a great book on Illustration that has a good section on Photography, retouching and manipulation, I think it featured Hypnosis the company who specialised in album covers (Pink Floyd etc). It's back in the UK so I can't give you the exact title etc. I did a small amount of airbrush work as well but it wasn't my forte, I was better with a brush.
For hand coloured work the two masters were James Wedge, (a book called Painted ladies) and particularly Bob Carlos Clarke - his best work is in a book, called Dark Summer. He used a combination of toners, chromogenic dye toners (bleach & redevelop in a colour developer with the relevant dye coupler added) and hand colouring using retouching dyes, the results are stunning. His two earlier books were also interesting, The Illustrated Delta of Venus (Anais Nin) and Obsessions.
I used to give occasional lecture/demonstrations on hand colouring in the early 80's, using toners & dyes and would turn a B&W image into an acceptable colour image in about 30 mins, cutting a few corners
This is the backing singer from Aphrodite's Child, Vana (also the name of her own band) - a quick hand coloured print made in the late 70's.