Read in order:
*Starship Troopers *by Robert A. Heinlein - a granddaddy of the humans v. aliens military sf genre. Bonus: powered body armor!
The Forever War by Joe Haldeman - humans v. aliens in a generations-long war, as the Earth soldiers become less and less connected to the society they are supposedly defending. Strongly influenced by Haldeman’s Vietnam War service.
Old Man’s War by John Scalzi - humans v. aliens with about a half-dozen delightful twists. More sarcastic and witty than the two previous books, but really, all of 'em are worth reading.
A short story that had an interesting take was Turtledove’s “The Road Less Traveled.” It’s an alien invasion but with a twist. The aliens, like many in the galaxy, happened on the key that let them develop interstellar travel very early in their technological development. So they grabbed their flintlocks and headed off to explore and conquer the galaxy. Boy were they surprised.
Don’t mention that near Barry B. Longyear. 
All of the BOLO books have to do with interstellar war.
I’m surprised no one has mentioned the Honor Harrington series.
Ah OK, thanks!
Thank you, I was getting it mixed up with this book by the same author which seems to have a similar premise.
Thanks again for the answers everyone. 
Oh Lord, I can’t believe I forgot the best one I’ve read in many years. The problem is that you don’t even find out that it’s a human-alien interstellar conflict until significantly into the book, so I’m going to put the rest of the post in a spoiler, including the title:
The Three-Body Problem. It’s hard-as-nails SF from China, with several jaw-droppingly beautiful scenes any one of which would be enough to hang an entire novel from. The way the book unfolds is amazing. Don’t be put off by the long beginning section about the Cultural Revolution; it sets up a central conflict for later in the book and the sequel.
The Honor Harrington series is all humans vs. other humans. There are a handful of other sapient races around, but none of them are particularly involved in the conflict (aside from a few treecats who have adopted Manticoran humans).