I know this is going to cause a back lash of disagreement but I cant hold back from replying.
For me, I disagree in buying any E-Book for information mainly because you will find the same information online within any good SEO site or a good forum like this one. So why waste money when it could be put it better use for something that isnâ€™t free?
Usually these e-books are not selling any secrets as yet again the same info can be found online unless a search engineer as gone rouge and he starts selling his firms secrets online *Cough* Matt *Cough* Cutts
Just for the record he isnâ€™t 'selling' secrets of Google from what I know
From some of the ebooks I have seen (not related to SEO) it explained how to resell the e-book that the reader had just purchased - that was the secret of making money on-line! I never read an e-book again when I read one of these
Sorry I know its not the answer to your question