Try "Sight Reading for Bass":Ron Velosky (BASSBOOKS.COM - BassBooks.com)
It's pretty good, and does what it says on the tin.
The two keys to reading are
a) build up slowly
b) practise
It's easy to learn that the line below the bass clef is bottom E, think you KNOW it and move on to more stuff too quickly, but it needs to be totally ingrained. Spend a week just reading and playing just low E/F and G. Practise it until the symbols on the page ARE the sounds - not just symbols which you interpret to mean a note name, which you then turn into a fret postition, putting your fingers somewhere, and finally making a sound. That entire process needs to be invisible.
Ian
I quote the book Ian suggested and I will combine it with Link Removed, great for pure rhythm reading. if you through them step by step, 5 page each every week, you will be able to read music in a short time and effortless