There are two book I highly suggest to learn and improve sight reading:
- Reading Modern text In 4/4 by Louis Bellson
- Sight Reading for the Bass by Ron Velosky
The first one is to improve your rhythm reading and the second to improve your quick sight reading. If you do both page after page, without rush and giving yourself the time to learn accurately you won't have any problem to read good and than you can move to the next step, working on real music charts, even in this second part you need to study first simple charts and than moving to complex ones, it doesn't matter if them come from different music styles, pop, rock, jazz, ballad etc etc, probably the diversity will help to improve your sight reading.
Never forget that sight reading is a true fitness, when you practice it every day you get it sharpen, vice versa you'll lose a little bit the speed in reading.
Anyway if you need any help feel free to PV. I have a reading class program and I will be happy to help you.
Cheers.
P.s. another good challenge is apply the sight reading to slap exercises.