Cromwell is a fascinating figure, and has had a lot of people writing about him. They often have channeled a lot of their contemporary experiences into what they wrote about him. Lord Clarendon , a thorough Royalist, thought he was talented, wicked man, Thomas Macauley , a complete Whig, saw him as the father of progressive England and called him Oliver Protector. Christopher Hill, a Marxist, was somewhat disappointed that he didn't keep going with a revolution once he started one. On the other hand, Antonia Fraser wrote a big whopping book that talks about his family and his temper and spins a rattling good narrative of exciting events and human drama and doesn't bother too awfully much with political theories and deeper aspects of the English Civil War, but though a catholic she doesn't whine about Holy Martyr Charles I. They're all worth reading...but there's only so much time. You'd likely find Fraser's in a library, well-thumbed. Used copies of Hill's God's Englishman can be found cheap in many second-hand bookstores, because it was often assigned reading- just make sure you look inside to make sure some nervous student didn't mark it all up with a highlighter.

There is actually a Cromwell Association which has a website with a good essay on biographies of the Protector by their President, Peter Gaunt, who also wrote a tidy little biography in 1996 that's also quite a good introduction. My only complaint about Gaunt's book is that it's not a long, rattling narrative like Fraser's, and he could well have written one. But if you read Gaunt's biography, you'll have enough time to go read the Putney Debates., and after that you will really want to go on and read more about the Interregnum in general.