I did a transfusion rotation at the beginning of my fourth year too... culminating in my decision to do AP only. 🙂 I didn't read anything in particular for the blood banking part; it's pretty basic stuff and you'll catch on right away. There are a couple of articles in NEJM, I think it was in 1999, on transfusion medicine that were decent.
If your rotation includes coagulation like mine did, the November 2001 issue of Archives of Pathology and Laboratory Medicine is devoted to consensus guidelines on thrombophilia testing, and I found it useful. Honestly, though, you don't need to read anything. Showing interest will be more than what most of the residents do, and if you pay attention, you'll pick up everything you need to know in a few days. There will also be books for the residents that you can flip through during your (extensive) downtime, and the attendings will probably give you some articles read.
Although I found it really boring, I don't think it's a bad idea to try out CP before starting residency. At least now I know it's not my thing, and I won't waste residency time on it before switching to AP only.