The non-fiction book I recommend most to fiction readers is Annie Dillard's Pilgrim at Tinker Creek. It's classified as nature writing but it's not, quite — it's a writer paying sustained attention to a creek and its surroundings over the course of a year and asking what the attention itself reveals.

Dillard's sentences are as crafted as any novelist's. Her argument — implicit rather than stated — is that attention is a form of respect, and that sustained attention to the natural world produces a kind of perception that is qualitatively different from casual observation. The book is an argument made through example rather than assertion.

I give it to people who read only fiction as evidence that non-fiction can do things fiction can't: it can be about something that is actually happening, that the writer actually saw, and the knowledge that it's real changes what the attention means.