The two I have are both very good, in different ways: TASTE OF A PLACE MALLORCA by Vicky Bennison, which is a mixture of authentic recipes and chapters on ingredients and where to find them, along with restaurant recommendations; and COOKING IN SPAIN by Janet Mendel, which obviously covers recipes from the mainland as well but includes Mallorcan dishes. I probably use this book more than any other as the recipes are very simple and often include very few ingredients (a plus in my eyes!).
I did notice, while we were in PP in April, that there was a rather nice-looking cookbook for sale (in English) in the gift shop on the Pine Walk - is the shop called Aurells, or something similar? I'm afraid I can't remember the title but that would be another one to look at.