Bill Baretta won't mainly because he is not all that recognizable yet and he is still fairly new to the Muppets. Wanted to get that out of the way first.
I think Steve Whitmire's would be interesting especially since he has gotten to see the company grow for so long. He was just a kid when he joined the Muppets and it was near the start of the whole franchise. Plus he has been very active in the Muppets (all of the films), Sesame Street and Fraggle Rock. Plus, he could discuss his relationship with Jim Henson and what it was like to take over his two biggest and most popular characters Kermit and Ernie. Mainly Kermit because those are HUGE shoes to fill. I feel he would have a lot to say and he is very smart and passionate when he talks about the Muppets.
Jerry Nelson's would be excellent as well. He has been working with the Muppets basically since day one. His book would be smart.
Frank Oz...Not sure on this. I would LOVE to see him come out with a book, but not sure how likely it would be. Atleast not very soon.
Martin Robinson...I've always wanted to hear more from him in interviews and things. I feel he is one of the most underrated and most over looked of all the Muppeteers. He has two great characters, but no one ever really thinks of that man behind them. Just not as recognizable I guess.
Fran Brill...now THAT would be interesting. It would be nice to read from a woman's perspective working with Jim. She was the first female Muppeteer (besides Jane Henson). She is also the longest running of the female Muppeteers and she has been with Sesame Street for almost as long as Carrol Spinney. She has seen much. Very smart lady and a brilliant performer (puppetry, voice over, acting...). Would like to hear from the female. She would have a lot to say.
Brian Henson...I would DEFINITELY read that and it would be a good book. Different because he could talk a lot about the company and the Muppets, but also give us deeper insight into the family behind them. How the two blended and how they didn't. His experience in taking the helm of the company. His relationship with folks at SW. So many things. It could be from any family member, but Brian's views would be interesting. That book should come out next. It is time.
As for human cast members (from SS):
Roscoe Orman has already released a memoir, I am pleased to discover. This will be great to hear things from the human actor/cast member's POV. Not much is ever really heard from them as people when not performing on the show. I think Orman's will be smart and great to read. I would like to read more, mostly from longer running cast members like Bob McGrath. Sonia Manzano could discuss more the writers perspective along with the acting too.
Joan Ganz Cooney is someone who I feel should write a book. I think I would rather read a book by her than most of the actual performers/actors. She created Sesame Street. It was her baby. I would like to get a deeper sense of her mission and goal with the show and how she came about creating it. What was her relationship like with the Hensons and the other performers? What does she feel about the direction the show is going now? It is one of the longest running shows in television history!
The possibilities are endless. I hope that Clash and Spinney have started a tradition that others will follow.