Is there any official published content centred around the plane Mechanus? I'd really want to know much more about the plane and its inhabitants. I feel inspired by the 5e Monster Manual, DM's Guide and Player's Handbook entries.
The supplement you probably want is "Planes of Law," which describes the lawfully-aligned planes of the Great Wheel in detail. Mechanus is, naturally, one of the planes it covers. You'll probably not find any treatment of the plane more detailed than this; It was released during the heyday of 2e AD&D's Planescape campaign setting, which expanded the planes from being an interesting place to visit into a full-fledged setting for campaigns.
Also... I'm not sure if I should include this or not, since it's not directly about Mechanus, but there is a second edition adventure (or rather, a series of adventures published in a single book) called "The Great Modron March." As the name suggests, it focuses on the Modrons: Those semi-mechanical exemplars of Law that inhabit the plane. It contains a little extra information about Mechanus that you might find useful, including its gate town (Automata), its inhabitants (chiefly the Modrons), its relationship with the rest of the planes (especially those in the Great Wheel), its Great Cycles (The giant clockwork keeps very regular time) and its Powers (assuming Primus the One counts as a power).