In the menu the book seller NPC puts up, it gives you a list of 4 upgradable stats, and beside each of those stats it says something like (0/3) That means you can get 3 upgrades of that stat total, once you finish one, the next in line becomes available. It all adds up to 9. On my bow it's 3 each for DEX and VIT, 2 for acc and 1 for skill speed. Each stat on the bow will get some sort of upgrade by the time its finished.