Is there anywhere in the game that tells you the order to open the books to Na-Krul's lair? I got it by trial and error, and I verified online that the order is correct. But is there anywhere in-game that tells you the answer, or were players supposed just to try all combinations?

Being a game from the 90s it's not unlikely that guessing was the intend

Yup,, looks like it.

Unlikely. However, I read one interesting thing - these books are actually present in the original Diablo (the quotes are present in DIABDAT.MPQ), but they are unused. They were used in Belzebub mod to open path to Horazon's Sanctum.

... aaaaaand he died with 2 fireballs. Wow.

Kaboom!