*This post has many spoilers in it. I recommend reading the book first if you havent. Its a fast read*
Enders Game is a science fiction book written by Orson Scott Card which involves a character named Ender who is tricked into exterminating an alien race. The book was written in 1985, long before Halo. There are many references to the book in Halo.
First, one of the most obvious is The Ender-Master chief reference. Master Chief is a super soldier, and basically wipes out covenant armies with little help. Ender wipes out a whole alien race in the book.
Another is that in the second book, when Ender is grown up, he has an AI named Jane which he communicates with through a Jade crystal on his ear. Jane is extremely similar to Cortana, the Master Chiefs AI.
In Enders Game their ships can travel at the speed of light. This means, if they fly to a planet at the speed of light for 15 minutes, its been 2 years for everyone else. Slip space could be a reference to this.
Yet another clear reference, is that in Enders Game the aliens are called "Buggers", and in halo, the drones are also called "buggers"
You may say Halo used the idea of the IF (International Fleet) taking kids while they are young and using them as supersoldiers. In Enders game, they take young children and train them to be soldiers. These soldiers are extremely good at fighting and know everything technical. In Halo, they kidnap kids and Make them into Spartans. See the resemblance?
Updating now, just giving you the option to read what i have so far