This should answer your question to save books: https://forum-narutoen.oasgames.com/page/show-post-24350-1.html

Yes, it is true that there's is a way to exchange it to make other skillbooks but not now at the very least. If you are willing to be patient until the update to the skillbook system we have, you'll be able to do so.

I haven't tried it with 100% confidence since I can't understand mandarin but there's going to be a Tsunade NPC for the skillbook system. To craft skillbooks, there's an instance via Tsunade where you can farm materials to use for the exchange system that looks like a slot machine. I've only used materials to craft a skillbook but never used a random skillbook to craft but the system was able to accept it as a material. As long as it doesn't give me back the same skillbook that I used as a material, I would be fine with it but knowing other games that have a crafting system it can happen -_-