Training wisdom doesn't come from a book. It comes from years of dedication and putting copious amounts of sweat and effort into your training.
While being well read or having a high-level degree in an exercise science will make you more intelligent about training, true wisdom only comes from years of practicing your craft.
Wisdom comes from trial and error. It comes from making mistakes and learning from them. It comes from practicing with a variety of different lifting styles and implements and learning what works for YOU.
Here are a few training truths only dedicated lifters really understand:
Almost anything works if you put time, energy, and effort into it. Low-repetition programs work. Medium to high-rep programs work. Linear, undulating, and conjugate periodization all work. Full-body workouts, upper/lower splits, push/pull/legs... they all work. The key? Your program only works if you put in the work.
You can have the greatest program in the world yet not see progress if you aren't being consistent and aren't working hard. Likewise, there are people out there with really crappy training plans that still manage to make progress through sheer hard work and dedication.
Obviously, a good training program will take you further, but effort and consistency are the only ways to truly get stronger and change your body.
This isn't to say that you have to train to failure all the time, or feel like you're about to start bleeding from your eyeballs after each workout, but you do need to work hard consistently. That's the recipe for long-term success.
If you're not having fun you won't stick with it. "Fun" results in consistency. Fun means you'll view your training as a "get to" not a "have to."
People can only grind for so long. If you're bored or unsatisfied with your program, or it's filled with exercises you hate, you won't be training for long. Find what YOU like and stick with it.
What's fun for some may not be enjoyable for others. Whether you want to lift heavy, get your pump on, or set your lungs on fire with metabolic conditioning, you have to make sure you enjoy kicking your own ass.
One of the best ways to prevent staleness is to become proficient with a variety of lifts. Not only will this keep your workouts fresh, it'll also help prevent overtraining on certain exercises which will keep your joints healthy and feeling good.
A good general recommendation for beginners (training for less than two years) is to rotate your big compound movements every six weeks. More experienced lifters can rotate their big compound exercises every three weeks.
While there are absolutely scientific principles of strength training and muscle development, understanding that it's also an art form is critical to learning how to move weight well. I'd even argue that lifting is more of an art than a science. Here's my rationale:
Science almost always follows innovators who try new stuff.
When a particular method works for lots of people, it gets studied so researchers can figure out what makes that method effective. What we know now comes from years of people just "figuring shit out" as they went along. It comes from the people who used trial and error until they got stuff right.
People are individuals who have different limb lengths and leverages.
While there are sound technique principles for each lift, there is no "one size fits all" technique for every individual. It's an art to figure out what works best for you and which cues allow you to best grasp how to perform a particular exercise.
Program design is largely an art form.
Not all people respond identically to the same program. We all have different strengths/weaknesses and muscle groups that respond faster than others. Figuring out the right exercises, frequency, intensity, volume, and tempo for your goal is a skill leaned over the course of your lifetime.
A seasoned trainer's intuition trumps the latest research study nearly every time.
If you want to learn more about training, you're much better off learning what works from experts who've spent time in the trenches working with ACTUAL people and who actually train themselves. The problem with a lot of exercise research? The sample of people used is generally untrained college kids because that's who the professors can readily get to do their studies.
We all make mistakes. It's part of the learning process. Nobody who has ever done anything great has had a linear path to success.
In the process of becoming a good lifter, you're going to make mistakes with your technique, programming, volume, frequency, intensity, tempo, etc. While you should strive to minimize your mistakes by learning as much as you can about training, they will inevitably happen.
If they're being honest with themselves, most lifters look at what they did in the early days and realize there was a much better and more simple path to get strong and build a good physique.
At the same time though, by making mistakes they've gotten a better understanding of the process of strength training and learned what effective training looks like for them.
As Thomas Edison said about inventing the light bulb, "I have not failed. I've just found 10,000 ways that won't work."
This point sucks, but it's absolutely true if you're pushing yourself. While you should be as consciences about your technique as possible, inevitably you're going to have a day where shit just goes south and you tweak, pull, or pop something.
Don't beat yourself up over it. Instead, consider it a learning opportunity.
You will now gain knowledge on how to work around injuries and facilitate your own healing process. You'll get the opportunity to learn about new exercises and techniques to get you back to kicking ass.
Consider it an opportunity to pay things forward and pass your newfound knowledge along to others who could learn a thing or two from the wisdom you just gained.