Yay! My department
Ive played guitar for about 7 years, but quit to play the bass, and have been happy ever since. Im glad you decided to play for pleasure, thats what its all about man. As far as a book, I cant really tell you one to get, I learned to play it myself. Here is a great site that can help you with the songs you like:
Bass
They have got alot of really awesome tabs there. Learning tablature is a good way to start. Learning chords and riffs takes some time. If you have a good sense of how to read tabs, and play them fast, improvising should come naturally.
The one thing I can tell you to do that is fun is the 5-7 stretch. This is basically the 5-7 fret on each string, if you jump 5,7,5,7 going down the strings starting from E(or b) at the top, going to the smallest string(g) at the bottom. Its a good finger workout, and if you like slap bass, its an awesome progression to play around with. Once you get fast at that, you will be coming up with your own 5-7s in different frets, and at different speeds.
If you want some more info, and a few more exercises, my PM box is always open.
Good luck with it, Im glad to meet another bassist.
Andrew