Depends on your interest, mine's in F1.
Any Autocourse will be a good starting point; there's an immersion of the entire season that is hard to replicate, short of driving these cars. I have every one from 1984 through 2010. Automobile Year was also an interesting read back when I was young; not sure how useful it is now; more emphasis on the industry's new cars and motor racing.
Pick up a driver's biography or two; I've read everything from Steve McQueen, James Hunt, Nigel Mansell, Niki Lauda, Peter Revson, and a few others. You get into the history and a viewpoint that the media doesn't always have access to.
My Favorite Race was an interesting one; it was a collection of 20-30 drivers' favorite wins from 1900-1973; you get an idea of how it was to drive and race in different eras of motorsport and in different disciplines. (I held onto it for nearly a year in my school library...when I realized I couldn't finish my year-end paper without borrowing more books, I reluctantly gave it back.)
Grand Prix Greats is one I find hard to put down; it's from the 1980s, and gives you some insight into 25 different Grand Prix drivers from 1930-1988 (some world champs, some really good ones that never were).
McLaren: History of Grand Prix, Indy, and Can-Am cars is a goodie; you see behind the scenes, it stops around 1987, but you get a great idea of what made a team spread out and tackle all sorts of racing series.
There's some other arcane books that I also have for reference like Formula One Constructors 1966-1986 and Grand Prix! Volumes 1-4, that are more stat-based but fill in the gaps of the mysteries between and during the races. And probably about 20 or so others that I'd picked up in the library when I was younger; lots of good stuff, but usually a few years old...was just as interesting and it really laid a foundation of the history of motorsports to me. After all, there's lots of stuff on the 'net about the present and recent-past, not so much about the historical aspect of it all.
Finally, I offer this
thread from The Nostalgia Forum of the Autosport website. Don't let the start date of 2000 throw you, it's always updated!