The tomes you buy from the store are BtA, not BtC.
When you are buying a tome, you can buy more than one of the same type, if you wanted to.
But if you previously bought a tome, it appears you can't buy another of the same type with the same character.
Please remove this restriction as it is unnecessary.
The reason I ask is because I was logged in on my main character and spotted tomes were on offer, so went to buy one for one of my alts, could I buy it, no, could I work out why I couldn't buy it, no. I even raised a ticket asking if the store was broke!
The guy who answered my ticket didn't know why I couldn't buy it so was unable to help me. Only afterward I worked out it seems to be that you can't if you bought one before.