And the simple way to implement this would be to just have the BTC +3/+4 tomes not disappear when they are read. They are BTC so know one else can use them, only this toon in a future TR.
That doesn't sound simple at all, compared to just applying the tomes benefit at the minimum level of each bonus, exactly how the LR/GR does it now.
"I require a reminder as to why raining arcane destruction is not an appropriate response to all of life's indignities" - Vaarsuvius, OoTS #674