the "last chapter" is already in the ending; there wasn't a plan for a fully grown book5 to begin with. the epilogue is there, and basically the game is complete, as in, it goes all the way to an ending. Marty had mentioned that he'll do some things which he feels should be added, changed, tweaked, and some longer time ago he also talked about wanting to redo, or rather, "touch up" book1, but don't expect major changes in the whole game "as is". MiTy declared it final, and they are actively pulling links to old stuff regarding FET from their patreon, or maybe are already done with it. they have new plans around the corner and seem to prepare for that. just few pages back there's some update post about it.