All of them. Seriously.
Sometimes one will show a view or have a comment on the exact thing you need where another doesn't.
I've got
Haynes Triumph 650 & 750 2-valve unit twins
Clymer collection series, BSA, Norton, & Triumph in one big book
Modern motorcycle mechanics - covers tons of bikes and general tuning too
and an assortment of factory literature
and a Triumph Chopper Guide (collection of articles out of old mags, mostly for entertainment)
and a buncha magazines
If you're going to get just one, get either the Haynes (cheap, easy to find and pretty good) or the factory manual.