I'm a big fan of 19th century fiction; these are usually very long books but they have short chapters so are ideal for the bus.
So my bus book is usually something by Charles Dickens, Wilkie Collins or Elizabeth Gaskell. It used to be George Eliot, but I've read all her books at this point.
This is the ideal bus reading; a lot of the time I'm speed reading the material for a seminar or meeting that I didn't read the night before.
At the moment, its 'David Copperfield' by Charles Dickens.
Edit:
I'm not interested in pretentious Lit here. I mean light reads you would recommend or otherwise - and why.
Contrary to popular belief, Dickens is actually very easy to read. He was the Victorian equivalent of pulp fiction. Well, sort of, he was a popular writer, not a 'high brow' writer ... Muggins... Anfor... take note!