With additive manufacturing, most users are still at the point of having more questions than answers. One interesting resource is from Fictiv, a service provider that connects users with the right additive or subtractive technology. The Fictiv Hardware Guide is a collection of articles from users that covers lessons learned as well as insights.
Some of the articles are in depth, some are geared to the newer 3D printing user.
For traditional manufacturing technology, you can find tips on cutting machining costs, and best design practices for CNC machining, as well as when to shift from 3D printing to CNC machining.
For additive technology, you can find information on the various types of 3D printing technology, frequently used materials, and printing tips.