What exactly is a cleaved spine?  Is that when the book separates its self (in half)? 

I received a book today that has lines in the spine showing that it's probably been read multiple times.  The spine is not separating, and the binding is still intact.  Will it be postable when I'm done?  I never bend the cover when I read, in fact, most of the time you can't even tell that many of my books have been read--lucky for the people who just got 3 Cups of Tea and the Friday Night Knitting Club!

I've received books with heavy creasing and lines down the spine and only if they are really bad is when it disappoints me... now, if its a few simple lines down the side that do not affect reading, then I'd say its fine :-)

As long as the binding's intact, it's postable.  Cleaved is when the spine is broken.