Paul,
The Fasola site is my reference book & very easy to remember, but for an introduction I prefer the Sacred Harp Singing pages that Warren Steel has put together.

But first, what have you heard, & what do you want to know. Depending on where you live or heard it 'shape note' can mean a couple of different styles of music. The Sacred Harp books mentioned so far are oblong and use 4 shapes. The 4 shape books were originally published 1800-ca1860. In the later half of the 19th cent. 7 shapes were used in oblong books. In the 20th cent. many regular hymnal shape books were published using the Aiken 7 shape system. These later 7 shape books tend to be more the southern gospel style, while the 4 shape books predate gospel & have a lot of folk hymns. The 7 shape oblong books are sort of transitional having some of both.

The Sacred Harp is the 4 shape book most have heard of & it is still used. The oldest convention will have it's 150th session in Aug. this year. The National Convention starts tomorrow in Birmingham & runs through Saturday. There are groups that gather all over the country to sing from it. The Southern Harmony, a slightly older book is also available & there are a few singing from it. William Walker published Southern Harmony, but when he switched to 7 shapes called his book Christian Harmony. There are 2 editions of Christian Harmony in use, principally in Alabama and western North Carolina. Two other 7 shape oblong books are New Harp of Columbia used in Tennessee and Harmonia Sacra, a Mennonite hymnal. Look at the singing schedules at the site I mentioned above for locations of singings.

If you interested in the 20th cent. 7 shape, I can't really help you. I know a lot of southern hymnals were published using the shapes, including the Broadman, a standard Southern Baptist hymnal.