There's certainly no shortage of information out there about Custer and the Battle of the Little Bighorn, it can be tough to find a good starting point. I'm gonna stick to books in my recommendations because unfortunately there are not many great films about the subject in terms of objectivity and insight. I'm also on mobile so am going to try to keep this brief.

One of my favorites is A Terrible Glory by James Donovan. In additional to a detailed account of the battle itself and its surrounding controversies, it provides a good deal of biographical information on several of the key figures involved in the battle. Donovan's writing style is engaging while still being deeply informative, and the book is very thoroughly researched and more objective than most. One of its best aspects is its overview of the official court of inquiry that took place after the battle to investigate what exactly happened and why. It's a great starting point for the subject.

Another fantastic book is Evan S. Connell's Son of the Morning Star. The author was primarily a novelist and as such the book is written in the style of one, but is still a work of non-fiction. It's an accurate retelling of the battle that is a nice change of pace from the dry slog that many other history books are.

A couple other detailed and respected books on the battle are John S. Gray's Centennial Campaign: The Sioux War of 1876 and Custer's Last Campaign: Mitch Boyer and the Little Bighorn Reconstructed.

If you're at all interested in Custer himself as a subject, I'd recommend Cavalier in Buckskin by Robert Utley, Custer by Jeffrey Wert, and The Custer Reader by Paul Hutton.

I hope this helps your foray into what is one of my personal favorite historical subjects.