Use coupon code TFRPLT4NFPT4. Here's a short description:
Liber Pro takes the 1961 Liber Usualis, a book of over 2,300 pages of Gregorian Chant for use throughout the liturgical year, and literally puts it at your fingertips on your iPhone or iPod Touch. Liber Pro adds to this tome a linked table of contents, user-defined bookmarks, vertical and horizontal paging and zooming, browser-like navigation, a dynamic liturgical calendar, and more.
Liber Pro is more than just an eBook. It is also the complete 1961 Roman Calendar, capable of calculating the moveable feasts throughout the year automatically. This means you can select a date; view the proper, common, and feria for that date; and jump instantly to the relevant pages in the Liber Usualis.