There is a very popular prayer book available called the Pieta Prayer Book. It typically has either a blue or orange cover and the image of Michaelangelo's Pieta on the front. The book is full of wonderful prayers and is very affordable. We sold several hundred when we first opened our store.
Unfortunately, we found that the prayer book also contains private revelations from people who haven't even been recognized as Venerable by the Church and there is no indication that these revelations have been in any way approved by the Church.
Update 10/8/12: Someone in the comments found a passage in the Summa that backs the position of Mutter Vogel so we publicly acknowledge our error. Still, the book contains unapproved private revelations so we will continue not to carry it.
While most are fine, there is one section that contains heresy.
The page with revelations from Mutter Vogel has this to say:
Certainly, the Holy Sacrifice is one and the same even when it's celebrated by an unworthy priest, but the grace called down upon the people is not the same.
Since we first opened our policy has always been "Good Faith. Guaranteed." which means that we don't carry many popular books including the Pieta Prayer Book.