This series was introduced to me by my mother as books that she read as a child (they were originally published in the 1940s, which totally dates my mom -- sorry about that mom :)
They are sweet, wholesome stories about three little girls, two of whom have been friends forever (way before BFF was thrown around) and the third wheel who turns out to be the perfect addition to their group. One redhead, one brunette and one blonde, the three are as different as their hair color and they have misadventures and get into trouble and discover things about themselves... you know, like almost all kid's books used to be back when "times were simpler".
The result is a wonderful trip into history and some life lessons that 7 to 10 year old girls (and maybe even some young boys who aren't to the "that's for girl's!" stage) will adore. Reissued by Harper Collins in the 90s, I believe there is a 50th anniversary edition out in stores now.