The Betsy-Tacy Treasury by Maud Hart Lovelace is currently on sale for $2.99 for Nook, Kindle, and at Kobo (and soon at the iBookstore). It's the first four books in the series in one volume:
Betsy-Tacy;
Betsy, Tacy and Tib;
Betsy and Tacy Go Over the Big Hill; and
Betsy and Tacy Go Downtown.
These are wonderful books for girls (or if, like me, you read them as a little girl and still re-read them as an adult
) )