by Jostein Gaarder is an ambitious project which falls flat - in my opinion, of course.
It is a very good introduction to European philosophy, with a few casual references to Eastern thought thrown in for the sake of comparison. Starting with Pre-Socratics, it provides a fairly simple and comprehensive look at classical philosophy. In the middle, it makes a detour into Christian theology and the Middle Ages before emerging triumphantly from the dark with Renaissance thought. Towards the end, it discusses Marxism in detail, and Darwin's evolutionary theory and Freud's psychoanalytic techniques as though they were "philosophies" (while many other path-breaking scientific discoveries are left untouched) before ending with Sarte's existentialism. It seems to be targetted at young readers, and may encourage some of the serious ones to take up the study of philosophy: if so, that much is in the book's favour.
As to the literary merits of the work, I have to regretfully give a total thumbs-down. The story is mostly dialogue; Gaarder uses the ages-old technique of Plato to get across complex philosophical ideas through relatively simple sentences. While the intention is admirable, the execution leaves a lot to be desired. Sophie comes across most of the time as rather moronic, and her teacher Alberto sounds like a pompous ass. Of course, there is some justification for the imperfections of characterisation, as Sophie and Alberto exist only in the head of Major Albert Knag who is writing their story: still, the overall responsibility as author lies squarely at Gaarder's door. Towards the end, the style of dialogue became so repetitive as to become grating: for example, the sentence: "a mere bagatelle, Sophie." is uttered like a chant by Alberto at regular intervals (to be totally fair, it may be a problem with the translation, but I do not think so).
Gaarder's idea to frame this (novel? -treatise?) as a "story-within-a-story", even though a laudable attempt, fails due to the total ineptitude of execution. Towards the end, as Sophie and Alberto "escape" from the book into independent existence in the land of imagination, the structure of narrative collapses like a pack of cards. The "Philosophical Tea Party" (a conscious take-off on Alice's Mad Tea-Party) reads like a cross between a scene from a play by Beckett and a movie by Bunuel. By last third of the book, the reader starts wishing for the end to come quickly.
I would recommend this book only for casual young readers who want an introduction to European philosophy. If they are really serious, I would recommend The Story of Philosophy
by Will Durant, which is a much better book and much more exciting.