Honestly, the only real way to make money from things like shirts is to have a stock of them printed and send them out yourself. Print on demand services like Cafepress, Spreadshirt and Lulu are more of a 'vanity' printing deal- 'oh my god someone's bought a shirt/book/mug/novelty teacosy based on my comic' vs. 'oh my god someone's bought merch and I made some cash'.
Unfortunately you need to be reasonably popular to make having shirts printed a clever venture- what a lot of people I know do is to take preorders and see how popular the design is, get a good number and take it to a shirt printers. If you're unsure of your numbers you can end up with a big box of shirts sat in your garage for years. A friend of mine who just did up a batch as shirts says that the minimum the printer would do was 18, and he waited til he had 25 pre-orders.
As for books, if you want to stay print on demand don't use Cafepress, their reported quality is a bit poor. There are other services like
Lulu,
ComiXpress and
Ka-Blam that have better reputations and print in colour.
A word of warning, though.. as a good basic rule to pretty much anyone starting out on Cgen, don't expect to make bank on your comic, especially not from merch, until you have a couple thousand readers. If you're really hoping to get some beer money though, a set of
Project Wonderful ads is more likely to work, but again don't expect much.
