Operating and maintaining an engine underway is a completely different animal than in a shop or a manual.
FF is correct in preventative maintenance is pretty textbook stuff. Based on a calendar or engine hour component and what many boat owners tackle...but when something stops working, or there is an alarm or sounds funny...they have to have someone else in the equation.
Repairing and tinkering with engines usually comes from over a lifetime of doing it, even in the PM only type stuff and picking up a little here and there along the way. It is as much about knowing what kind and how big of an oil leak is OK till you get around to it, or where that sound is really coming from and what takes to fix it, and even what tools and how to use them correctly to get the job done.
Please don't get me wrong....but the books are really best for their trouble shooting or explaining why a problem has occurred or is reoccurring. Without sitting the book down next to an engine (or you having the engine or component clearly in your minds eye), reading about it I am not sure has enough of a transfer till you roll up your sleeves.
Bottom line is as has been suggested, start working on stuff and following a manual, or take a hands on class, or hand around a friend who does it all the time.
My son was a tinkerer from the toddler stage...grew up building and rebuilding computers in his room, got a job out of high school as a mechanics helper and within a couple years went from computers to rebuilding heavy equipment and the entire asphalt plant with just minor guidance...picking up required skills such as welding along the way. In just a few years he skyrocketed past me and most in knowledge of all sorts of complex mechanical designs. He wound up becoming the IT guy for a very large construction outfit because of position and pay, but management still sends him out on mechanical repairs because of his troubleshooting and wrench turning skills. So it doesn't take forever to learn...but you do have to immerse yourself at some point to pick it up quickly.
Those books are good...but I suggest you beg, borrow or steal some trouble shooting tools unless you have them and start fiddling with everything from alternators to pumps to complete small engines along with those books as soon as possible. If you cant do all that, use a mechanical friend or befriend someone you can be a gopher for to gain experience