Bookworm

In a nutshell: make words out of the letter tiles and get points for doing so. But don't think you can just get away with lots of three-letter words (like And or The), because shorter ones produce burning fire tiles which will set the library on fire if they reach the bottom of the screen before you can make words out of them.

The absence of any 'against the clock' element makes this a somewhat leisurely puzzler if you like that sort of thing, though there's also the option of a pay-to-download Deluxe edition which has an 'Action' mode, 'Save game' option, extra graphics, sound, and music, plus the ability to make words up to 12 letters long, assuming you can think of any of that sort of length in the first place...

Mouse click on adjacent letters in the correct order to make words (click on a far-away letter to restart a new word), click on "Submit" (or double-click the last letter, or press Return or Enter) to claim points for them.

If you're stuck, click on the bookworm to scramble all the letters - though this also increases the number of fire tiles.