The best introduction to Bruno Latour as a philosopher wasn't written by him: it's the book Prince of Networks, by Graham Harman. Latour was never formally a philosopher, but his work was massively concerned with metaphysics all the same.

Harman described Latour as a "secular occasionalist". Occasionalist philosophers are concerned with how anything can interact with anything else (a much broader and, to Harman, a much more interesting question than how subject can interact with object, the question which has dominated Western philosophy since at least Kant). Occasionalism sees all substances as radically isolated from each other - but then have to posit a special exception to the rule of separation, like "God" (in classical Islamic occasionalism) or "eternal objects" (A.N. Whitehead's more recent occasionalism).

Latour dispenses with any special substance in occasionalist philosophy by positing that anything can potentially take on the role of mediating bridge between anything else. Harman doesn't view this as a perfect solution to all the problems of philosophies of occasionalism by any means, but he views at as innovative step, worth pursuing further, which for Harman entailed the development of a version of "object-oriented ontology" - but this post is about Latour as a philosopher, so I'll stop now.