There's a few good natural breakpoints scattered throughout - and the book is composed of three 'books', all of which make EXCELLENT save points (I used them as such). However if your lifestyle affords it, my opinion is that books and games are much better to swallow in one lump. It shows you if the developer/author is doing something cool (Dune, Operation Flashpoint) in which the context/mechanics switches completely in a fairly natural manner, rather than plodding through the same system explored to the point of nausea (Half-Life, Neverwinter Nights).

For me, GTA3 and Hitchhiker's Guide (the book) stand nearly alone as systems so diverse that they can be explored almost infinitely - and in which the context switch is controlled by the viewer/reader.