I'm currently re-reading 'For the Kings Favor' (also published as 'The Time of Singing' abroad), about Roger Bigod II, Earl of Norfolk. His life's story has a lot of parallels to the Marshal's, in fact, they were contemporaries- and the Marshal does appear in the book a few times.
I just read her duology on the Marshal and I have to say she takes the dry factual entries of the likes of Crouch among others and makes it palatable for just about anyone with an interest in things medieval. I read both books at least three times so far. Good reading...I imagine I'll make it through all of her books at some point, or a good number of them at least.