When I get a textbook I can't use it in between missions or in when I start a new mission.

Is the WAD?

You can just apply the textbooks to your characters. You can do it between missions, at the end of missions, or during the missions. It won't work if the solder is already fully ranked up in that area (indicated by a triangle in that area.

The problem I have is that it doesn't appear in their inventory after a completed mission. I have it in stock, I just can't use it.

Puppy look it up on the MGS: PO+ FAQ topic I just posted, that should explain it.

Ok first of all erhmmm... lets say you've completed the last stage of your mission and you know your FOX unit soldier has a textbook so go in the menu that allows you to select thse soldier fro the sneak team and click on the desired soldier that you want to use a textbook. Now you see his inventory there is a textbook unequip it with "Triangle-> Change equiment" and right after this press "Triangle-> Use Item" scroll down until you see the desired teatbook you want to use on your soldier.

Btw: has your character get's exp he's stats my level up too like exemple if CQC was C it will level up too B but it won,t happend if there is a little triangle in the corner

Ps:my english isn't good