http://shmuplations.com/yoshi/
This short-but-sweet interview with producer Shigeru Miyamoto provides some rare insight into the development of Yoshi’s Island. I’m not sure why, but other than this interview, Nintendo never gave any developer interviews in either game magazines or strategy guides. My best guess is that Nintendo’s tightly controlled PR decided it was best to leave some room for the soon-to-be-released N64.
In any event, this interview has some good info about the design choices in Yoshi’s Island, as well as some commentary from Miyamoto on the previous Mario games.