It's a horrible idea. The book was written by the Spanish equivalent of CAIR, and the press that published it, owned by the Marists, is ultra-liberal and announced that it was doing this to encourage Islam in Spain. Catholics in Spain are, needless to say, pretty upset by this idiotic remark.
Furthermore, the government paid the entire cost of these books. Not only are there way more books than needed, but they are going to be sold through the mosques, which will be allowed to keep the entirety of the purchase price. Books for Catholic religious ed, on the other hand, are paid for mostly by the Church and are sold through school and other regular book stores, with no profit going to the Church.
Religious ed is offered as a weekly class in public schools in Spain, with an ethics option for kids who are not religious. Something like 85% of Spanish families request it for their kids; however, in the last couple of years, as an attempt to discourage kids from taking it, the Socialists have removed its grading and standing as a regular class.
Zapatero is famed for his attempts to promote Islam. He is a leftist atheist himself, but he sees Islam as a way to attack the Church, which he regards as his arch-enemy. The title of the book, by the way, is "Getting to Know Islam"(in Spanish, of course) and it is obviously designed for use not just for Muslim students but for some future "introductory" classes on Islam for non-Muslims.
Well, then it is a bad idea.
When the conservatives get back in power, they should consider my idea. :-)