I've got one of the Carmichael books (The Lance Armstrong Performanc Program; 7 Weeks to the Perfect Ride), but it doesn't go into enough detail -- it only briefly describes the various techniques. All it says about threshold is that sometimes "the aim is to stay aerobic and stay under the threshold", but he doesn't really explain why.

One thing weird about that book is that Carmichael says that if someones spends more than (i think) 45 mins aboive their threshold, they need at least 2 days recovery rides, but his 7 week program recommends hard days on Thurs, Sat and Sun, then 'spin' days Mon Tues Wed.