I would assume that, given the long history of Bill Gates being intimately involved in many aspects of Microsoft, that he has a particular talent for taking written material and absorbing it at a very fast rate (look for Joel Spolsky's story about Date functions in Excel).

That's the sort of skill which never diminishes. I bet Bill G can absorb a relatively short book at a very fast speed, especially if he's on vacation with large blocks of reading time available.