Tunebook for iPad is now identical to the Tunebook application - don't buy both! I keep both on the App Store because it's not possible to provide an upgrade path to a single app without forcing some users to pay twice.
Tunebook is a viewer for music expressed in the ABC format. ABC is a simple text-based notation for music and many thousands of tunes are freely available in this format on the Web. ABC is also supported by many music applications. See http://abc.sourceforge.net/ for more details on the ABC format. The format is primarily geared towards traditional folk music where tunes are short and fit on a single page.
Tunebook allows you to import files in the ABC format from websites into its database. You can then list the tunes, search for tunes by words in the title, by key and by time signature. Selecting a tune will display it in musical notation and allow you to play it in a variety of voices. You can also create tunebooks to organize your tune collection.
Tunebook is based on the well-known abc2ps application, fluidsynth, and the GeneralUser GS soundfont. It is licensed under the GPL. The source code is available from http://www.jhlabs.com/tunebook.