Normally, games built around a single mechanic aren't very good. Generally the designers run out of fresh ways to abuse the concept about an hour into the game, and as a gamer we're left with 10 more hours of rehash, possibly with a timer thrown in for artificial difficult.
Bookworm Adventures, on the other hand, is amazing. It helps, certainly, that the central mechanic (boggle) is so much fun, but Popcap managed to stretch it in ways that would make 10 tons of silly putty jealous. By occasionally adding simple embellishments, the designers open up tremendous new gameplay opportunities. The main quest can take about 20 hours, but the gameplay always seems fresh.
It helps too that doing well in this game makes you feel smart. This is a challenging game, especially later on, and it forces you to dig deeper and deeper into your vocabulary well. Despite the challenge, however, the game rather cleverly helps you when you're struggling. For every enemy you defeat, you gain experience and levels, which grant stat boosts. You keep these levels even when you die. If you're in a particularly tough spot, if you persevere, you'll eventually get through thanks to better levels and your own improvement.
Finally, this is a charming, funny game. The premise is completely ridiculous, but Popcap decided to play it straight and it works really well. Now, due to industry regulations, I am required to end this review of Bookworm Adventures with the following phrase: Sometimes the pen really is mightier than the sword.