Sophie's World goes through philosophy from beginning to end, accurately depicting a reasonably large amount of beginner philosophy; obviously, once you've read it, you aren't exactly a master philosopher, but it's a good starting point for anyone interested in philosophy, and an even better starting point for someone who isn't.
While the book has some wonderfully insightful and clever moments and a great depiction of philosophy, the actual story was... A little less likable.
The plot promised a huge and amazing ending, and what it ended up giving was an anti climax with a the very out of place "Invisible People" and the revelation that Hilde's mysterious, deeply thought out plan was, in fact... Just a friendly prank against her dad.
All in all, definitely worth reading despite the crappy story. However... Just skip over the disaster that was "The Garden Party."