YY to everyone who loves HP. Me too! And agree about Sir Terry, particularly anything with Tiffany Aching (If you haven't read The Wee Free Men, I strongly recommend it).
Also agree about Laurie Graham. I've read several, but particularly recommend a book called "The 10 O'clock Horses", when you start reading it, you think you hate the narrator (the book is told from his perspective) and it turns out that he's just so lovely.
I also LOVED the Rosie Project - an easy and very funny and sweet read.
A bit more 'meaty' is The Time Traveller's Wife. It's a beautiful book.
But my book that I enjoyed most, and have brought for at least 10 people has been The Night Circus. If you like the magical worlds that JKR or TP imagine, then you'll enjoy this.
And I agree with everyone who said teen fiction. But I like child fiction too. Old stuff - Judy Bloome can be funny and (for teens) poignant. Anne Fine. Jacqueline Wilson. Malorie Blackman. The Hunger Games series. The Artimis Fowl books. Anthony Horrowitz (sorry about my spelling)
If you like crime and don't mind gritty then Ian Rankin writes a cracking book.
And I adore Kate Atkinson. Behind the Scenes at the Museum.
Happy reading. x