I originally pronounced it Tee Eye Book, but the issue with that is that people miss the t, and hear iBook, so I reformed at started calling it a Tie Book.
Yes, "Ti" itself is an elemental abbreviation, but in the context of this particular PowerBook it was a truncation of the word "Titanium" (which, by coincidence is the elemental abbreviation). Since it's a truncation, you pronounce it like the word being truncated. Tie-Tay-Nee-Um.
Thus, "Tie-Book." Plus, the "cute" shortening of Titanium PowerBook was supposed to rhyme with iBook which was also available at the time.
Yes, "Ti" itself is an elemental abbreviation, but in the context of this particular PowerBook it was a truncation of the word "Titanium" (which, by coincidence is the elemental abbreviation). Since it's a truncation, you pronounce it like the word being truncated. Tie-Tay-Nee-Um.
Thus, "Tie-Book." Plus, the "cute" shortening of Titanium PowerBook was supposed to rhyme with iBook which was also available at the time.