Alice climbs through the drawing-room mirror into a country laid out as a chess problem, where she is a white pawn who must reach the eighth square to be crowned. She reads Jabberwocky out of a book printed backwards, runs flat out beside the Red Queen only to end where she started, hears Tweedledee, prompted by Tweedledum, recite the poem in which the Walrus weeps over the oysters he is eating, argues with Humpty Dumpty about what words are allowed to mean, and meets a White Knight who cannot stay on his horse. You do not need Wonderland first. Under thirty thousand words, and set pieces rather than plot — colder and more argumentative than the first book.
Through the Looking-Glass is 29,561 words. At an average adult reading pace of 300 words a minute that is about 1 hr 39 min, or roughly 1 hr 14 min if you read quickly and 2 hr 28 min at a more deliberate 200 words a minute.
That count is measured from the complete public-domain text rather than estimated from a page count, so you can compare all the books side by side.