We had a variety of simple books for our children to read. There was one special book that they liked very much ("A Ghost named Fred"). When the child could read it without help, we inscribed his or her name and the date. They were so proud when their names were written in the book.
And I might add a proper way. I've seen parents spend 10-15 min. using flash cards on 6 year olds. Probably 2 to 5 min is the max. Its a lot better to read them a good, illustrated book with big print and let it come naturally.
I can remember when a friend of mine was surprised that I was not using flash cards to give my daughter a head start for when she would start first grade in the coming year. She was doing that with her daughter who was a friend and the same age. I said I did not think it was worthwhile. And, sure enough, when they both entered first grade the two little girls were on a par with each other. The whole business of "pushing" is something I don't approve of. There is a proper time for everything.
I agree with "allmymarbles". Don't pressure her, she is only 5 and 5 year olds naturally have shorter attention spans. Give her time. Maybe try playing a game with the letters to make it more fun and interesting.
http://pbskids.org/games/letters.html?vm=r
Good luck and have fun!
Give her some time. Children's attention span seems to improve at age six. Don't pressure her. That will only make her anxious.