The improvisor's Bass method, by Chuck Sher is the most valuable book I have ever studied. Plus it has transcriptions by Paul Jackson, Jaco, Mingus, Ron Carter, Jamerson..... There is alot of great material, well explained, and usable by all skill levels, upright and electric. And then there is the Simandal book. All classical bass students know this one and groan. Piston's book of Harmony is the one my mom gave me, with Paul Des Marais accompaning workbook.