Nicola,
What a lucky you are that your mom took you to Hungary.
I were just few days in Budapest, long ago, but I have to
come back soon because I liked it very much.

I wouldn't make it wider that 12. The maximun I have built is 30 cm wide (= 11.8 inches). In the picture of the instructable you can see at the botton shelve, just in the center, 12 big (art history) books that have lasted there more than 5 years without problem.

But use a thick hard wood for the shelves. Otherwise it could be that they bended down.

Good luck.