Machinery's Handbook. Stuffed with practical information about materials, shop practices, etc etc., all kinds of info "beyond theory". Spend the money & get the big print version.
For motion control topics relative to a mechanical engineer, I have used the "Smart Motion Cheat Sheet". Find it on the web in PDF format and download it. I also have used SEW Eurodrive's German "Practical Engineering" guides (can be gotten in most Euro languages if needed). Must go to the Eurodrive German electronic documentation site, dig for it, and download in PDF. Lots of practical, rigorous, motion control engineering methodology applicable to a lot of projects. A lot of the same material can be found in stepper & servo motor catalogs, ball screw catalogs, etc.
Convince your boss to spend a few hundred dollars on some inexpensive PLC equipment (automationdirect.com or similar) and just play with it: on/off actions, sensors, lights, programming, wiring & wiring components. There's nothing like creating sparks and blowing stuff up to learn about how them electrons flow through wires.
Try to get a subscription to Machine Design, Cotnrol Engineering, Design Engineering, and any of a dozen other trade rags that are out there. Sometimes they actually have some good articles.
Start showing up at your local professional society meetings and learn about "networking".
And besides the previously mentioned efunda.com, try
There is a lot of great practical engineering information and methods stashed away in vendor catalogs. All for free, usually available for download. One of my favorites is the INA Bearings catalogs.
TygerDawg