Second vote for Gilman's "Illustrated Guide to Pruning". (as well as his online stuff - search "UF Ed Gilman" and you'll hit a jackpot)
Also +1 for Shigo materials - "A New Tree Biology" isn't so "new" any more, but still very good...really essential.
Beranek's "Working Climber" DVD series is very good.
Jim Urban's "Up By Roots" has a lot of good information about soils - especially urban soils. It is a little heavy, but worth plugging through.
Pascal Pirone has a good book on diseases and pests
UC's Abiotic Disorders of Landscape plants is good too.
Are you an ISA member? If not, join. Arborist News and Journal of Arboriculture are both good publications...maybe not every article, but there is plenty there to keep anybody learning - far less intimidating when it is digested a few pages at a time.
Go to educational seminars put on by local ISA units, Extension offices, State Forestry agencies, etc....