Man, Daniel Jensen is so underrated, yet every time I mention him to people who've actually read his stuff, they light up. The one that consistently gets brought up is 'The Silent Chapter'. It's this quiet, devastating historical novel about a bookseller in post-war Europe, and I swear it has paragraphs that just hang in the air after you read them. It's his breakout, won a bunch of awards, and seems to be the gateway for most readers.
His sci-fi duology, 'Chronos Divide' and 'Chronos United', has a much louder fanbase online. The world-building is dense and philosophical, not your typical space opera, and the fan theories about the ending of 'United' are a rabbit hole all their own. If 'The Silent Chapter' is his delicate literary hit, the Chronos books are his cult genre classic.
Honestly, I bounced off 'A Catalogue of Small Regrets'—it’s a collection of linked short stories, and while critics adored it, I found it a bit too precious. Still, it's always on the 'Also by' list, so it must have its audience. For a newcomer, I'd say start with the silence, then dive into the time war.