Another "I can't pick just one" post- so I am going to turn this into my favorite books (note the s ;))
Anything by Sophie Kinsella, loved the Shopaholic Series and I also really like her other books too- very light and fun.
Harry Potter Series. OBVIOUSLY. Who doesn't love the Harry Potter books? I think my favorite book is Harry Potter and the Goblet of Fire.
Charlotte's Web! A Childhood favorite book. I think I read it 100 times... something about cute pigs! Can't resist!
Anything by Mary Kay Andrews! All her books make me want to take a vacation to the deep south and collect things. I really recommend "Hissy Fit" it was the first book I read by her!
Marian Keyes- Sushi for Beginners. Loved it!
Cecelia Ahern- PS I love you- way better than the movie!
Hester Browne- Little Lady Agency (And then all of the subsequent "little lady" books!)