The Trouble with Series

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I'm still debating over what to buy with my Fictionwise money, and here's part of the dilemma...some of the books I want are parts of a series but I haven't read the first book yet, though I own it. I don't see buying the second if I've not read the first.
Or maybe I get to read the first book in the series, but never get to the next. This has happened with Larissa Ione's books, Colleen Gleason's, Rachel Vincent's, the second set of Virgin River books. I...