Okay, I get it. We can do anything we can imagine. What does that have to do with marshmallows? Nothing, really... unless you're in Rowboat Watkins's head.
Most of the enjoyment I got out of this was from looking at the pictures. They're pretty fun. I just wish there'd been more of a story to go with them. Think of the possibilities! In this book, though, we're just told about marshmallows. They live in families, they learn how to be squishy in school, and they don't dream at night (interesting fact... but not much to build a book around). Marshmallows do, however, have imaginations, so they can have dreams of the waking kind where they blast off in rocket ships or face down dragons.
Aside from the illustrations, this book is a bit weak. Even so, it's worth a look. You might get a chuckle out of the marshmallows going about their daily lives.