I was asking myself this same question almost 2 years ago. The best way to learn how to play the bass is just play the thing (reading all the theory/instruction books in the world won't actually make you a better player, maybe more educated,diverse and well rounded ie Flea doesn't know the difference between an F# and an F Clef). Try to play along w/ radio, cd's, w/e gets your goat. Check these
Gen theory links and
this book list. Maybe go see what your library has to offer. If you want to cheese it out, download some tablature of your favorite bands. But it's all depending on how serious you want to take the instrument. Learn to read music in the process, listen to music you don't normally listen to, exploit this site (and it's search engine

) and your favorite internet search engine to the fullest.