I can recommend three or four albums to start with. Begin with Then Play On, which bridges Fleetwood Mac's transition from straight-ahead blues to more pop and rock-orientated efforts. If you like what you hear then try Bare Trees, which is an early 1970s post-Peter Green album that spotlights Christine McVie and newcomer Bob Welch.
You might also try Pious Bird of Good Omen, which showcases the original band playing great covers of Elmore James, Robert Johnson, and others, and features the original version of "Black Magic Woman." It also includes "Albatross," which IMO is completely different from anything else they did. It's a gorgeous tune, and according to legend served as the inspiration for Lennon's ode to Peter Green, "Sun King."
Finally, you might want to check out Live at the BBC. It contains both covers and originals of blues tunes, but also includes contemporary (i.e., ca. 1967-1970) rock and pop songs, lively but sloppy covers of 1950s rockabilly and doo-wop that will probably drive some crazy, and Jeremy Spencer's dead-on imitations of Elvis and Buddy. Great fun if you're in the mood and approach it the right way (not too seriously). The album concludes with "Man of the World," which gives a harrowing description of Peter Green's mental condition but is a terrific song nonetheless.