Alright, so I see this question pop up a lot. The core reading order for 'Invincible' isn't actually books—it's the original comic series. The main run is the 144-issue 'Invincible' series by Robert Kirkman, Cory Walker, and Ryan Ottley. That's the spine of everything.
You can jump right into the three 'Invincible Compendiums', which collect the whole saga. They're massive, but they're the most complete way to read. The story is linear, so just go Vol. 1, 2, 3. Some later spin-offs like 'Guarding the Globe' or 'Invincible Universe' weave in, but honestly, for a new reader, just stick to the main compendiums. Get through those first, then worry about the rest if you're still hooked. Trying to read everything in 'chronological' order from the start is a headache you don't need.