Experience. Make lots of mistakes. Build things. Read. A lot.
I would also add, ironically: No 3D printing.
It's very interesting to see young engineers do such things as specify, for example, an array of 0.250 in holes spaced 0.750 in apart and require a tolerance of 0.001 in.
You then take that same engineer into the shop and ask them to drill just two of those holes.
The surprise in their expressions when they can't produce a single good pair of holes is very telling.
3D printing doesn't teach you about mechanical design and manufacturing any more than a word processor teaches you about good typesetting and layout.
So, I'd say, design things and make them by hand using hand tools, manual milling machines, manual lathes, etc.
Just drilling a single precisely positioned and accurately dimension-ed round hole can require an hour of work, if not more. There's a reason professionals call it "hole manufacturing" rather than "drilling". You need at least three tools to manufacture precise round holes.
Anyone who thinks that drill bits are for making round holes needs to go do some research.
Impossible to boil it down to one book. So many disciplines.
I would suggest you get a copy of Machinery's Handbook and study it.
There are also books that are collections of mechanisms. I probably have ten of them. There are some online resources today with animated versions of mechanisms. Google is your friend here.
Learn about materials. What's the difference between mild steel, hot rolled steel, stainless steel, 6061, 7075 and MIC-6 Aluminum. What do you use, when and why?
Research topics such as "hole manufacturing" and "high speed machining".
Read online forums such as "Practical Machinist", ask questions.
Learn about Geometric Dimensioning and Tolerancing.
If you don't have a background in Physics, learn some. Mechanics and Heat would be the basics. Don't need Calculus for the fundamentals.
Plastics is a topic in and of itself. Study injection molding. Visit sites like Protolabs, read through all of their materials. Understand their process. Learn about conventional injection molding.
Learn about analyzing structures/designs for strength, deflection, etc. Do this by hand first with simple shapes.
Learn FEA (Finite Element Analysis) and apply it to both mechanical and thermal analysis of designs.
Understand how to use fasteners. You'd be surprised how many people don't understand such basics ideas as "you never use a screw for positioning, only for clamping".
Read about finishing: paint, powder-coating, ceramic coating, electroplating, anodizing, porcelain enamel coating, chromate, passivation, texturing, sand blasting, brushing, chemical etching, etc.
Learn about laser and water-jet manufacturing processes and when to use them.
If you have access (or the money) get yourself in front of a milling machine or lathe (or both) and learn to take parts from CAD to finished product. Not trivial. Don't need a big sophisticated machine to do this. A mid-size bench-top mill and lathe will do. No CNC. Manual first.
Design and fabricate all the parts needed to convert your mill and lathe to CNC. Now learn CNC machining.
There's more, lots more. For example, adhesives is a huge topic. Lubricants is another. Composites. Casting. Extruding. Sheet Metal. Die casting. Etc.
Yet another huge topic is understanding all of the above in the context of costs and manufacturing efficiency. What's the consequence of designing a rectangular hole with 0.005 in radius corners on an aluminum part to be machined? When do you switch between extruding and machining and why? When do you completely redesign a part in order to better fit the best manufacturing process? What are the implications and manufacturing realities of requiring extreme accuracy?
You are not going to pick all of this up in a week or a year. It takes time, years. I've put my hands on most of the above during my career, but I am talking about 30 years designing and manufacturing all kinds of products. Don't expect instant results.
That, BTW, is why becoming a Solidworks "driver" does not make someone a mechanical designer. Solidworks is simply a design documentation tool. Becoming good at driving the software will not provide anyone with any of the knowledge listed above.
Better done in the context of actually working rather than tinkering. You don't have to become an expert at all of these either. Almost no such thing any more. Yet, it really pays to have as wide an understanding of manufacturing as possible. 3D printing is an almost insignificant segment of the totality of manufacturing.
I would also add, ironically: No 3D printing.
It's very interesting to see young engineers do such things as specify, for example, an array of 0.250 in holes spaced 0.750 in apart and require a tolerance of 0.001 in.
You then take that same engineer into the shop and ask them to drill just two of those holes.
The surprise in their expressions when they can't produce a single good pair of holes is very telling.
3D printing doesn't teach you about mechanical design and manufacturing any more than a word processor teaches you about good typesetting and layout.
So, I'd say, design things and make them by hand using hand tools, manual milling machines, manual lathes, etc.
Just drilling a single precisely positioned and accurately dimension-ed round hole can require an hour of work, if not more. There's a reason professionals call it "hole manufacturing" rather than "drilling". You need at least three tools to manufacture precise round holes.
Anyone who thinks that drill bits are for making round holes needs to go do some research.