An answer in this interview, but not the one we wanted...

Reddit: The books from Savage have started to become a clutter on player inventories. Is there any plans to add these to a currency tab similar to beast tribe currencies?

Naoki Yoshida: Adding new items to the currency menu is not something I want to do without proper consideration, and books from past raid tiers have very little practical value. I'm sorry to deliver this answer, but I’d prefer players to just discard them.

On a more serious note, currency items clutter up character data, and if we create currency data for each item, it would further increase the amount of data transfer required, which may cause latency issues.

I totally understand these might have sentimental value and it's hard to just throw them away, but if you could please just keep the books from the latest raid and get rid of the past ones...
So, yet another deep-rooted issue with how the game handles inventory. Keep the raid tokens as inventory items, and the system doesn't care how many you have or not because your bag size is the same regardless. Turn them into currencies and you have more data to carry with you at all times.