Cool, we're on the right track now. You really don't NEED a book for this. Studying transcriptions of Jamerson or (any legend) can't hurt, but you'll essentially be just doing what I describe below with a bit of a head start.
The best thing for learning how to create bass lines is your ears. Listen to lots of songs in the genre you are working with and hear what other are doing. Very basic I V country/polka lines? Melodic McCartney lines? Bouncy, funky Motown lines? Punchy violent metal lines? (Eventually, listening to multiple genres will help you alot, but to get the hang of it, just focus on what you're specifically working on.) Just listen a lot and focus on what the bassist is doing. I think you'll find some very common elements within a genre. Once your ear is full, take one line and either transcribe it or learn it yourself. Make sure you understand the supporting chords that you're working against. At this point you can bring theory in to see how the bass line compliments/contrasts with the tonality of the chords. Listen to the rhythmic approach: playing with the kick drum or more with the hi-hat or ride? Do this with another song, repeat the process and build yourself a bass vocabulary of other people's stuff.
Now take a song that you want to add a new line to and try out some of the stuff you've picked up to see what fits for THAT song and what doesn't. After a while it becomes second nature - you won't even think about it most of the time. The challenge can be making a line sound unique from another one (always a struggle), but they don't all have to be different if it's serving the song. Take 100 bass players, have them build a new line for a song and you'll probably end up with about 75 versions, depending on now creative and skilled the bassists are.
I work with an original jazz fusion band and write about 90% of my bass lines. I always start with simply following along with roots. As I get my root fingering patterns established I start embellishing - adding the fifth, adding a third, adding a seventh, whatever the section of the song calls for. Then I hone in on how I'm going to lock with the drummer. After playing lot 2 or 3 times, I usually have a pretty passable approach and just work on polishing it up. I've only been playing bass for 3 years, but do have a considerable music background (including tuba), but this I not rocket science. In some ways, bassists can get away with a lot and not every line needs to be show-stopping bit of brilliance. Sometimes root whole notes is what is called for - sometime silence is even better.
There's lot of online sources to help with analysis (Scott's bass lessons features a regular presenter (Danny 'Mo') who essentially just does bass line analysis for popular songs - it's an appropriate topic for your needs.
Just listen a lot and absorb - this will come quite quickly.
