I assume that if I control a card or ability that says that I can't lose the game, such as The Book of Exalted Deeds, it is theoretically possible for my life total to go into negative?
It is possible for your life total to drop into the negatives regardless of such effects. All those effects achieve is that they prevent the state based action that makes you lose the game cannot do so. Hardly any game where a player wins through damage is won with the opponent going to 0, rather it is usually won by dropping the opponent to less than 0. (Going to 0 is enough to make the player lose the game, though.)
Yes, and not only theoretically. Even in normal cases, a player's life total can become negative (C.R. 107.1b), but usually, a player with 0 or less life loses the game as a state-based action (C.R. 704.5a). However, if a permanent you control has the ability "You can't lose the game and your opponents can't win the game", you won't lose the game this way (C.R. 101.2).
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)