Most textbooks have about a three year life cycle before new editions are released.
Notifications of new editions usually come out about six months prior to the new edition being available.
New editions are usually released in the middle of the school year. Publishers do this knowing schools are often still using the books which keeps demand up for their remaining stock, but leaves professors, students and Bookstores in a difficult position if the remaining stock is sold out.
Once a new edition comes out (and sometimes before), they stop printing any of the old edition. When they run out of the old edition, they are permanently out.
The cost to Bookstores for new editions are typically $10-$15 more than they were for previous editions.
Publishers often increase their costs for current editions by $5-$10 each term.