Any book with this on it:

Dr Seuss is fantastic for that age. Great illustrations, memorable phrasing, and fun adventures for the kids. The messaging is also good, ranging from listen to your parents (Cat in the Hat) to standing up for what's right (Horton Hears a Who) to being environmentally responsible (The Lorax).

"Happy Birthday to You" might be a fun birthday gift if she's got one coming up.

I don't know if you read the New York Times, but this article is a must read for anyone with kids learning to read: http://www.nytimes.com/2010/10/08/us/08picture.html?_r=1

Humor is the best way to inspire an interest in reading, I recommend:
Click, Clack, Moo: Cows that Type (Caldecott winner) and Giggle, Giggle, Quack...both by Doreen Cronin

I'm the Biggest Thing in the Ocean by Kevin Sherry

Chester by Melanie Watt

I also recommend,
When Dinosaurs Came With Everything by Elise Broach
How Rocket Learned to Read by Tad Hills

Junie B. Jones
It a series about a girl about 6 years old and is funny, easy to read, and not at all "Princessy". They're easier than Beverly Clearly (who is great, but maybe a little tough for a 6 year old to read) but sort of have a Ramona feel to them. I loved them when I was young and the girl I used to nanny for loved them. They were a huge incentive for her during a very tough struggle to learn to read.
Not to mention that I still found them endearing and clever when I read them to her.

If she can handle simple chapter books, I would start with a boxed set of Laura Ingalls Wilder. The earliest books are very simple to read, have occasional illustrations, and the great thing is that she can grow with the series--they get more complex and deal with more serious subjects as the protagonist grows up. Also, the books are historical and endorse some pretty fantastic values.

I still own my childhood copy of Richard Scarry's Best Word Book Ever. The text is simple and east to read, and the illustrations are wonderful.

I love Calef Brown's books: "Polka Bats and Octopus Slacks" and "Dutch Sneakers and Flea Keepers". They're poetry (with great illustrations, but offbeat in a way that kids really like. They're especially fun for parents and kids to read together.

Daniel Pinkwater's books are fantastic, and cover everything from simple picture books to young-adult novels. His chapter books, like the Blue Moose series, might be perfect.

For the more advanced reader, Ursula LeGuin has a series of chapter books called "Catwings" that are great for kids who want to read "real" books. The books, and the chapters, are short, so they are good for intermediate readers, but LeGuin respects her audience, and the stories are thematically pretty sophisticated.

My 7 year old daughter loves to read the Charlie and Lola books.

Personally, I think you can't beat Beatrix Potter. The original Peter Rabbit stories are easy to read, the pictures are lovely, and they're sweet.

Frindle is a really fun kids' book. It IS a chapter book and I haven't been six or been around six-year-olds for a while, so I'm not sure if that's too much.

But it's a great book about words, how they come about, how we decide what words are "right" and "wrong." It's the kind of book I think a mom or dad would enjoy reading to their kid as well, so it might still work even if the child's solo reading skills aren't especially strong.

For picture books, one of my contemporary favourites is Donna Jo Napoli's Albert, about a shy guy and how he's eventually tempted to venture out and enjoy life. It's cute and beautifully illustrated.

Kid's graphic novels are so great for that age, because they are starting to want slightly more complex stories, but many of the interesting chapter books are a bit beyond their reading levels. My daughter loved the "Bone" series by Jeff Smith. Hell, I love the Bone series. Super cute; good adventures and loveable characters. Also "Rapunzel's Revenge" by Shannon Hale. Not at all princessy in the traditional sense. A super kick-ass female character. We also liked "Magic Trixie" and the "Asterix" books.

Blueberries for Sal. It might have been read to the child already so they are familiar with it.

Curious George

For a girl, the good old Betsy Tacy books are great because as the main characters (who are five years old in the first book) age, so does the reading level. The Betsy Tacy books set during their grade school years are great for young girls and in a few years, when your girl is in 6th grade or so, she can revisit the series and start in on the high school/college/marriage books, which are young adult fiction.

We introduced Mo Willems and Chris Raschka to our beginning reader. If you like classics, Robert Louis Stevenson's A Child's Garden of Verses, the Amelia Bedelia books, Eric Carle's books, Roger Duvoisier's Veronica, Maurice Sendak, especially Where the Wild Things Are and In the Night Kitchen and Louise Fatio's The Happy Lion.

A good book for a 6 year old REALLY depends on the six year old. Where is she at in "learning to read." Can she handle a simple chapter book?

Does she like animals, fairies, people, machines, poetry?

I would love to give you lots of ideas, but without dialing in a little, I have too many......