“Burn” is a commonplace synonym for oxidization, that is, adding an electron. Likely the GP is “eating the books, and thus is warmed as a consequence of the Krebs cycle.

We can be glad for GP that these days it is uncommon to encounter a book with a CD or DVD attached. I’m not sure those are safe to eat.

Semantics dear Watson: it's the inner joy that's internal.

Never let an explanation ruin a terrible pun.