Jesus asked Maria Valtorta to write on January 28, 1947: “The work that was given to the readers by little John [as Jesus calls Maria Valtorta] is not a canonical book. But it is always an inspired book that I give you to help you understand what my time as a master was like and to get to know me.”
For me, this statement from Jesus means that Valtorta's books do not replace the Bible or represent a new gospel. Maria Valtorta's books describe the biblical representations in more detail. The Bible, especially the New Testament, is sufficient to understand and believe in the life and great deeds of Jesus Christ. You don't need to read Maria Valtorta's books.
Personally, I find these books to be very lengthy, but also very enriching, and as Jesus himself writes, it helps me to better understand what his time on earth was like - to better understand that he was on earth like you and me. Breathing, eating, talking like we did. Because of the very precise descriptions it allows "to see" with the inner eye how Jesus performed these incredible miracles, with whom he had contact, how he spoke to people, how he reacted. I find all of this incredible and a grace to have access to this material. It gives me deeper insights and is therefore very enriching for me - I can only recommend it.