If you want some history and background then this is a perfectly good introduction to Gregorian Chant. Unfortunately I bought it expecting to the be more of a tutorial on the technique of singing chant, which it definitely isn't.
It's a good introduction to the development of chant and has a representative selection of examples of the chants used in the liturgy, with recordings of the Monks of Solesmes.
Unfortunately, the spoken text does let this product down somewhat. Sarah Moule is reading from a script, but it doesn't need to be made quite so obvious - she's painfully slow in her rendition. It would have been much nicer to have someone talking with a little more warmth and a slightly faster pace, rather than the over careful enunciation that's served-up. If that doesn't put you off then this could be a good purchase.