Carol Kaye is one of the greats of bass playing. She played sessions on over 10000 recordings, rock, funk and jazz. Along with James Jamerson, she is the very definition of Motown bass.
Her site is a very useful resource, especially the
Education section.
She provides hundreds of useful tips for the intermediate to advanced bassist for free, but she also advertises her instructional books which I have not seen, but sound good.
In addition, the site provides a good guide to Carol's career, recordings, pictures and so forth.