I think you can expect a solo album with just him and his guitar rather than a biographical book about him.
Mark was and still is a very private person. Privacy is sacred to him, so I don't think he's interested in writing a biography in which he opens up his life to the world.
And if he does, then we'll probably only hear the old stories e.g. "blue up the family radio, if you drop this I'll drop you" etc. and not much new. But who knows?
What I could really imagine is that he will present us more songs of autobiographical content. Songwriting is his thing, but spreading out his life in a book not really.