Interesting. I kept all my engineering and physics text books from college with the intention of MAKING my children learn the the content before I send them to college

Punishment with college text books (evil grin)
...statics, dynamics and material science seems to be where you are headed. Engineers are born IMO, but I believe you CAN train yourself to be an engineer. I know a lot of engineers and had a lot of friends in engineering school that were not born to be engineers...some of those guys are probably better doing something else, but yet they are engineers. The point I'm trying to make is that you don't have to understand the the entire range of engineering to be a useful engineer. If your job (John) is to determine or validate form and structural designs from an engineering perspective, you can train yourself.
You will need to start with some basic statics. You can pick up some used statics and dynamics books and start reading. Some freshman or sophomore level physics books are good too. They will cover mechanics on a lower level. Start there. You need to understand the fundamentals before you start cracking the topic specific books.
In engineering school we went through that same process. You don't want to get ahead of yourself and over look anything. A good engineer needs to consider all the parameters, and it may be difficult to so without understanding the fundamentals. The lower level physics books will help there.
Once you've familiarized yourself with the basics, then you can hit the "big" books. You will learn and appreciate the higher level concepts and designs easier. Most higher level books will touch on the basics at the beginning, but IMO those sections should only serve to refresh the memory for those who had already learned the concepts.
So! Get yourself a physics book, then a statics and dynamics book (or books as the two are generally taugh separately) and finally a material science book.
Visit or have someone you know visit a college book store and buy some used books. You'll probably spend a couple hundred dollars, even for used books! (rememeber the expensive engineering books, guys?)
Most text books have an accompanying solutions guide. Get that too. It will help you with the problems. You have to perform calculations and go through the problems yourself. You can't start to think like an engineer until you have gone through the thinking process required to solve the problems. Engineering is more problem solving than anything else. Creativity is also critical, but not knowing the problems one might encounter in a creative design can lead to expensive solutions...or rather mistakes.
Onces you start to understand engineering concepts and start to think like an engineer, FEA will be a powerful tool! Since you already have that at your disposal (I've seen some of your designs and they can easily be imported into an FEA program for cruching), you should learn faster than the average engineering student. I had a difficult time with some concepts, but after college and after having experienced the real world, a lot of the concepts were much easier to understand. They "clicked!"
You're in a good environment at your current job to teach yourself enough engineering...and do it fairly easily. You have to be dedicated as the books will bore you. Use what you have available (software at work) to make learning fun. Try to solve the problems or at least see the results using your programs to illustrate the lessons.
In engineering school, it was always difficult for me to see the results because I couldn't see it in the real world. Some of the most successful students seemed to absorb the lessons without questioning the application. They just took the lesson for what it was and were content with arriving at the correct answers to problems. It's sort of like faith. I'm not too keen on faith

, but the lessons, once I found the application for it in my head, were better engrained, and so they became a part of my problem solving skill set....experience without the experience you might say.
There! Just some misc. ramblings to get you thinking.