Connie Carl at Rainbow Ranch, was only a single volume and not part of a series, published by the Goldsmith Company in 1939. However, another Goldmith book which was not written by Millie, Girls of Silver Spur Ranch, uses the same dust jacket art leading one to think perhaps they were part of the same series, however they appear to just have the same type of western theme. Shown below are two scans taken from eBay. The one on the left shows the art mock up for the Connie Carl book and the one on the right shows both Rainbow Ranch and Silver Spur Ranch. Rainbow Ranch was written under the pen name of Joan Clark, also the same pen name Millie used for her Penny Nichols series, also published by Goldsmith.