John, I don't know you and you don't me (AFAIK) but if you happen to notice that I blogged about your geekiness, please understand this is a compliment. It is, in fact, such a compliment that I imitated you and put together a similar (altho simpler) spreadsheet.
After entering 2007's books that I bought on Amazon (title, price I paid, current kindle price if available) and doing some calculations (assume I only buy the kindle edition if it is cheaper, which is not necessarily an accurate assumption because I might now pay more to not have dead wood cluttering up the place), I will note that I spent a little over $1700 on books for myself last year, and switching to currently available kindle editions would only have saved my about $114, implying a payoff in about three and a half years, longer than John. It's worth noting that 2007 is _after_ when I made the decision to buy used whenever possible/cheaper, which may explain some of the weirdness, altho I think more of it is explained by the massive non-availability thru kindle of my kind of reading material in general.
Along the way, I spotted several more anomalous missing books. The second book in Jack Campbell's Lost Fleet series, for example, is not available thru kindle, altho the first and third are. Mysterious. Also, _Fire Study_ was just released and isn't available on kindle. I'm going to e-mail amazon about that because I want to read it. Now. Which is the real payoff of the kindle.
Altho now that I look at it, the release date is actually March 1. Hmmm.