Man, my kindle's been practically running on fumes lately, so I've been combing through a lot of the free stuff. The absolute standout for a freebie has to be 'Mountain Man' by Keith C. Blackmore. It follows this alcoholic loner named Gus who survives in the Canadian wilderness after everything collapses. It's less about massive hordes and more about the brutal, grinding reality of staying alive alone. The dread is so thick, especially in the first book, 'The Hospital'. I remember reading a scene about him scavenging in pitch darkness and having to pause just to breathe. The audiobook version is often free with Audible trials too, and it's fantastic.
For something more focused on community rebuilding, 'The Last Survivors' series by Bobby Adair is a solid pick, though the quality can be a bit uneven. The first book, 'The Last Survivors', sets up a world decades after the initial fall, where society has reverted to a kind of feudal, plague-fearing state. It scratches that itch for seeing how new societies form and fail under that kind of pressure.
Honestly, a lot of the best free ones are first-in-series hooks, so you get a taste and then decide if you want to invest. Project Gutenberg is also a weirdly good source for older, public domain takes on the apocalypse, like 'The Purple Cloud' by M.P. Shiel, which isn't zombies but has that same end-of-the-world isolation vibe. It's a different flavor, but the loneliness hits just as hard.