A good books helps. I learned a lot from Sloan's, but now keep a copy of Zinn and the Art of Mountain Bike Maintenance handy. The reality is it's become a little used reference, since there is so much info online these days, much specific to components that simply don't fit some generic mold any longer.
For "tweaks" though, you're going to only get good at that by doing it and learning what a turn of the barrel adjuster affects or how to square off the end of a section of housing or how much spring tension is enough when balancing the left and right brake arms.
But yeah, a good book is a good start...