Liber Pro is described as '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' and is an app in the Education & Reference category. There are four alternatives to Liber Pro for a variety of platforms, including iPad, iPhone, Android, Android Tablet and Google Chrome. The best alternative is Chant Tools, which is both free and Open Source. Other great apps like Liber Pro are Breviarium Meum (Paid), Evangelizo (Free) and Universalis (Paid).
It is mostly intended to be useful for containing the Gregorian Mass propers as well as the Gregorian Mass ordinaries as found in the Liber Usualis, and transcribed on GregoBase (http://gregobase.selapa.net) and there may be some mistakes in the chant.
Universalis Publishing was started in 1995 with the aim of harnessing computer technology to help in enriching the spiritual life of Christians, specifically by making the liturgical and devotional resources of the Catholic Church available on the new electronic media.