Hello Antonia

I have no ability to evaluate your sentence, but if I were you I would write it in a way like;
On the Book Day, 22 April 2003, they published a paper entitled "Who reads how many books in Croatia", which reported the result of a survey on the frequency Croatians had got in contact with books in the twelve months of the year 2002.

Antonia - I'll have a try.

On the occasion of Book Day on April 22, 2003, researchers published Who Reads in Croatia and How Much, reporting the results of their study of the interactions Croatians had with books during the year 2002.

Or, if the study considered newspapers, magazines, etc. as well as books,

"...the results of their study of Croatians' reading habits during the year 2002."

Your original phrase, "personal contacts with book" (it should be "books") suggests to me that maybe Croatians don't read books, but they do buy them to display on their shelves, or prop up their wobbly tables, or some other kind of "contact" that doesn't include reading. 

Antonia wrote:

I didn't know that you can collocate ''frequency'' with ''have got'', I've never heard it before.

If we want to be more correct grammatically, the sentence should be like below, though it sounds a bit awkward.

On the Book Day, 22 April 2003, they published a paper entitled "Who reads how many books in Croatia", which reported the result of a survey on the frequency (with which) Croatians had got in contact with books in the twelve months of the year 2002.

paco