ATTENTION: Don’t get screwed.
The base price of the book is $14.99, or $4.99 for the ebook. Some minor variations may occur in your own currency, but by no means should you have to pay just south of 50 euros.
UPDATE: The ridiculous price issue should be fixed. If you still encounter it somewhere, let me know.
Select the book option from the Buy Stuff pulldown menu for marginally more information.

Or, if you’re feeling particularly nutty, go to either the EU corner of Spreadshirt or the US version to buy stuff that seemed like a good idea at the time. Select the Merch-page from the pulldown to do your shopping (EU) right here.