You cannot change the brightness of eInk readers. The 600 has a display that is less readable and has more glare than the 505 or 300 as it utilizes the touch layer for navigation. The touch layer is some kind of a plastic layer which sits right above the eInk display.
The 505 and 300 don't have such a layer. Navigation is by buttons only.
I've read that readability of the 900 is supposed to be better than on the 600 allthough the 900 has a similar touch layer like the 600.
If you still can return the 600, go out and try the 900 if you look for the same functionality or for the 300 if you just look for book reading and can omit some of the 600s functions. Most of us who own and love the 600 like the device mostly because of the functionality, most times not bothering with the drawbacks you have to deal with.
I hope I could be of a little assistance.
