Quale ti è piaciuto? = Which one did you like?
Quali ti sono piaciuti? = Which ones did you like?
The person asking the question here assumes you only enjoyed one of the two books, so it would be strange to use quali. However, if there were more to choose from (Which of the books in the library did you like?) then quali would be more appropriate.
OK - I know (in simple terms) when we translate 'to like' we use the reflexive form of piacere, and translated literally it means 'is pleasing to'. So, surely, the answer to this question could also have been "Which of the two books pleased you?" - as well as "Which of the two books did you like?" Confused here - I always think of 'like' as being 'pleasing'.
Mukkapazza above gave the clue...Quale = Which (one)...Quali = Which (ones )...It follows that the past participle should refer to one book (un libro) which is masculine and singular...hence piaciuto....If you had a similar sentence..Which books did you like... it would become..Quali libri ti sono piaciuti. Hope this helps.