You may also want to check out MySQL and InstantDB. MySQL is probably a bit medium sized, but popular enough to have books in Barnes and Noble.
http://www.mysql.com/ I used InstantDB - now SimpleDB - to convert a db system from Access to SQLServer. InstantDB could read in text and also allow for SQL functionality. It was easier to update then SQLServer so it turned out to be a perfect stepping stone.
http://www.simpledb.org/ If it isn't free, it may be cheap and worth a look. You'll have to do the research though, as my knowledge of it is out-of-date.
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I'd use MySQL, if only b/c the user-base is so large.