Well, the first book on a dedicated ereader, that I bought and read as opposed to free and flick through (Alice in Wonderland) was either
Back to the Moon by Homer Hickam, or
Black Rubber Dress one of the few available mysteries when I bought my Rocket eBook at the end of 1999.
I worked in the dotcom world back then so my first Rocket purchase was a subscription to
Industry Standard. Hey, I still had 18 months before all my stock options totally imploded.