"Enclosure" is due out April this year (to coincide with his exhibition
at Yorkshire Sculpture Park), although I notice amazon.co.uk already
lists it for sale.

144 pages, covering Cumbrian sheepfolds, sheep paintings and other wall-
related art. Priced £32 in UK (amazon.co.uk is quoting £19.76).

See http://thamesandhudson.texterity.com/catalogue/2007spring/?pg=22
for more details. I haven't found any indication of a matching US
release from Abrams.