IMBA-UK is now selling a fully updated version of classic trail building book Lightly on the Land on its website. The 304 page book concentrates on the nuts and bolts of many technical skills such as building with rock, felling trees, building with timber and bridge construction.
IMBA’s own Trail Solutions is probably a more suitable trail building primer, and Natural Trail Surfaces by Design (also available from IMBA) will tell you more about designing and laying out trails. But this is the book for you if you’re interested in the actual construction aspects. It’s £29 (or £25 if you’re an IMBA member) from www.imba-uk.com.