The Studio Handbook by Martin Kinnear.
I attempt to paint like him but I think my work is just a bit too light weight to match up to his skills. Never the less I have learnt an awful lot from his courses some of which I have applied to my own work, but I have to say it deflects me from my main thrust which is light and happy and dare I say just a bit frivolous. Jack Vitriano says he keeps away from other art because he doesn't want to be pushed off course, and I kind of get what he's saying.
However this little handbook by Martin Kinnear is very useful, full of technical information and a great addition to my studio.