The Trouble with Reading on my iPhone

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I have about 40 books on my iPhone right now. All of the ones on my Kindle app were free, and a number of the ones on my eReader app were, as well as Stanza. I got quite a few classics, the Harlequin freebies, and all the Samhain titles Kindle is offering free.
The problem I'm having is that I'm reading three different books, one on each application. Then I'm reading a paperback. I've become ADD reader!
I don't know how to take them off once I've read them...