Nobody ever wrote a 25MLOC program from start to finish, so I don't think it makes much sense to read it that way.

I'd read it the way it was written: from the beginning. What's Linux 0.01 look like? What's the next changeset after that release look like? What was necessary to add the driver for your favorite device? What changes were made for your particular CPU?

Programs are not static works (except maybe TeX and Metafont). They exist in the form they do in order to be amenable to changes. So look at the changes that drove it.