I noticed a lot of search engines had pages leading to the books but all of them are now "no longer here" pages. Several had auxiliary pages mentioning that it could be preordered around May, 2007.

My guess? It was for sale but was pulled later when contracts expired. Amazons pre-order for Dec. leads me to think that MacMillian will release it again as a new issue.

(shrug) Just the way it goes, I suppose. Just because an ebook takes no space and can never degrade while sitting on an online shelf, it can still disappear due to the wiles and ways of business.