The amount of text in books is surprisingly finite. My best estimate was that there are ~10¹³ tokens available in all books (https://dynomight.net/scaling/#scaling-data), which is less than frontier models are already being trained on. On the other hand, book tokens are probably much "better" than random internet tokens. Wikipedia for example seems to get much higher weight than other sources, and it's only ~3×10¹⁰ tokens.