I should say that they have made a really nice collection of songs. Some are easier to play than others, and I personally wouldn't have gone with pictures of sheep as a cover photo, but the content is quite nice. I really like the different kinds of songs that you get to play, the notes are quite good, it has has lyrics for the songs in case one is drunk enough to start singing while playing the ocarina, it might be difficult, but one can always try... :)
It's a beautiful collection, of some really nice songs, with a great deal of detail gone into the preparation of such a fine book. However, don't buy it as a teaching book, buy it as a song book.
It also contains a brief introduction to the Irish history, some information on how to play the 10-hole ocarina, and a very small introduction to the music theory.
Almost all the notes have key positions below them, in case you were inclined to use them.
Also, some songs have keys for both, an Alto C Ocarina, and an Alto G Ocarina, so you can play them as a duet, though i'm not sure why anyone would want to... Sounds like a lot more work... :)
But you have the option of trying both out and see which one sounds better to you, on your ocarina, with its particular keys.
Also, some of the songs, can be found on Johann Rotter's (the author's) youtube channel. So you can also see how they are meant to sound.
Overall, a really nice book, and a really good, rich and diverse collection of some wonderful Irish songs.