Your lower back should remain braced and stable under load. Foam rolling it will encourage movement and instability which will eventually cause damage. Your upper/thorastic spine should be nice and mobile though so roll away on that, just don’t go below your lowest rib.
The grid rollers are the best all round rollers available. Look for one with a hollow plastic core and foam on the outside. The solid foam ones lose their structure quickly so don’t bother with those.
The rumble rollers are effective at unmatting particularly tight spots, but they’re not as useful as a grid roller.
There are quite a few copies of the grid roller now — this one looks great value with free delivery. http://www.amazon.co.uk/PhysioWorld-Grid-Foam-Roller-33x15cm/dp/B00ITQGCC2/ref=sr_1_2?ie=UTF8&qid=1401705891&sr=8-2&keywords=grid+roller
As has been said, a lacrosse ball is brilliant for getting into really tight spots (or 2 taped together).
Have a look on http://www.mobilitywod.com for loads of ways to use the roller and lacrosse ball.