Learn to sight read; almost without exception, teachers of and books about jazz are going to be dealing with a lot of functional harmony and the assumption from the start is going to be that you can read music. Learning to read is one of the few things that you don't need a teacher to work on.
I learned to read in high school by starting in beginning band (which was reading/playing rhythm rudiments with a pair of drumsticks) and then adding the pitches in the next level, which was playing with the big band (no small ensembles at my high school). The book that I spent the most time with when working on my sight reading was
CC2008 Rhythms Complete , it starts simply and moves in a very consistent and progressive way to more complicated rhythmic phrasing.
Then just start grabbing stuff and reading. A guitarist buddy of mine said he used to read clarinet exercise books just like he was reading the newspaper. Read through trombone etudes, cello etudes, pretty much anything and everything you can find in bass clef.
My sight reading isn't something I've kept up, but it has stood me in good enough stead that I can read most treble clef melodies, most written bass parts etc that I get handed.
