I was going to suggest that.
John Ringo’s Legacy of the Aldenata series.
Four main books plus some side stories.
Very good military SF
Plus you can get all four books for free at publisher’s website, Baen Books. The Wiki article has direct links for download.
Legacy of the Aldenata - Wikipedia
Robert Silverberg’s The Alien Years
Re: Worldwar
I would stick with just the first four set in the 1940s. The quality degrades from there. The Colonization books aren’t bad per se but I am not sure I would recommend them and the epilogue novel is just bad.
An interesting take on the idea of an alien invasion is Roger MacBride Allen’s Ring of Charon. In this book the aliens aren’t arriving from anywhere else, they’re already within the Solar System, it’s just that humanity’s actions have given the ‘aliens’ their wake-up signal.
This may well be outside the OP’s limits for what they want to read - in a very real sense the largest part of the conflict between the humans and the ‘aliens’ is stunted and stalled, because neither side can easily get beyond their own preconceptions towards an accurate understanding of the nature of the other side. There are no ‘alien’ characters, for example, because the ‘aliens’ simply can’t be thought of as individuals, let alone individuals that the audience could relate to.
One of the reasons I liked the book (and the follow-up, The Shattered Sphere) is that much of the plot hinges upon the protagonists answers to the questions, “What is life?” and “What is intelligence?”
I will second many of the recommendations here: Footfall, Worldwar, and The Legacy of the Aldenata (The invading Posleen are, IMNSHO simply the antagonists, not the villains of the piece. Though quite sufficient to give anyone nightmares.).
Some of my favorite books with aliens interacting with humans are post-invasion stories, where the war has been lost and the aliens have conquered. Timothy Zahn’s Blackcollar books; Eric Flint and K.D. Wentworth’s The Course of Empire* and The Crucible of Empire; and Gordon R. Dickson’s Way of the Pilgrim.
*Link goes to the book’s page in Baen eBooks’ Free Library where, with the permission of publisher and authors, anyone may read online, or download, the ebook version of the volume.
I read [noparse]http://variety-sf.blogspot.com/2011/08/fred-hoyle-john-elliot-for-andromeda.html[/noparse] A for Andromeda (pub. 1962) so long ago I barely remember it, but it obviously made a strong impression. Warning: the link points to a synopsis which may be an unwanted spoiler.
Patient was over long ago. I did like the series up to that point
He also co-authors the Looking Glass Series with Travis S. Taylor, which kicks off with an alien invasion when a scientists accidentally creates wormholes to “other” places, and he also has the Troy Rising series, which was partially inspired by Howard Tayler’s Schlock Mercenary web comic.
Granted, the Troy Rising series never has a direct alien invasion on Earth, but the Horvath do KE strike three large cities by way of saying “High!” And then park a battlecruiser in orbit and say, “Nice planet you got there, monkey boys. It’d be a real shame if a rock or something dropped on it. Now, in exchange for our benevolent protection, let’s talk about your stockpiles of heavy metals…”
Genesis. The Earth was a peaceful, pastoral planet and then humans just showed up overnight, took over and started trashing the place.
In honor of Mother’s Day: My mom kept the family bible “on the shame shelf as the Oz books, with all the other fantasy fiction” (her words).
This thread prompted me to go find the ebook so I can re-read it. Baen has it, DRM-free. 
Most instances listed so far – Worldwar, Footfall – involve non-humanoid invaders. “Humanoid” is quite a restriction in this genre.
How about Invasion of the Body Snatchers? The Pod People are humanoid-by-imitation, that’s why they’re scary.
In that genre, I’d also suggest The Puppet Masters by Robert Heinlein.
IMO Niven has written at least two better invasion stories than that, namely “World of Ptavvs” and “Protector.”
And just seeing the title of this thread made me decide to read Heinlein’s “Puppet Masters” again.
I got a malware warning from Chrome on that page.
How appropriate!
Baen doesn’t believe in DRM, nothing they sell has DRM. That is why they get my money. Fictionwise can burn in hell. [Fictionwise has a bad habit of unlisting books, or whatever you call it. I bought 20ish Pratchett novels from them, and they no longer list Pratchett at all. Thankfully I backed them up to cd, memory stick and external HDD …]
Good idea for a thread, I love a good alien invasion story.
One to avoid: (this was posted by me on another board in a thread discussing the worst books we had ever read, prior to this my vote was for *Ice Station *by Matthew Reilly)
Well, Thrintun are not humanoid. Pak Protectors, OTOH . . . well, I won’t spoil what they are, but it’s definitely humanoid.
That sounds interesting. Thanks!
Joe
You’d best be prepared to invest a lot of time in it, though. Turtledove’s storytelling-style is usually unbearably tedious, like, “And then another locust came and carried away another grain of corn . . .”