Different people react to the written test differently. Some find it incredibly easy without studying, while other's struggle through the test after days of studying.
If I remember correctly, I read through the book once or twice, and passed the test with 100% (ok, yes, one question was marked incorrect, but my answer was the most accurate, and I have the handbook to prove it). Keep in mind, I took the test a year after I took driver's ed. in High School.
On the other hand, I know people who flunked the test numerous times after days of studying, and they had just gotten out of drivers ed.