One question, answered from the books
When should I invade?
Less often than you want to. The invade is a play that spends your structure to attack theirs, and it only profits when you already know three things: where he is, that your lanes can back you up, and what you will take if he does not show. Invade to punish information you have, never to generate information you lack. And when you are the one invaded, the answer is not a duel at your buff: do not die, counter the opposite side of his jungle, burn his time, and return to your rotation. One camp is never worth a fight; the trade you want is two of his for one of yours.
The decision, as a picture
What it looks like in a real game
An enemy Camille once ran straight into my jungle to throw me off my opener. All she actually did was let me stay on my sequence while she wasted her own time; I was still on schedule for the Dragon, one camp down. That is what a bad invade buys: the invader donates tempo to the person with the cleaner system.
The concept: Counter the opposite side
The Strategic Jungler gives the invaded jungler a complete script, and Start Disciplined supplies the accounting rule underneath it: never fight over a single camp, because the map always offers a better trade than the duel.
"When invaded, do not die."
"You sacrifice one buff but gain two of theirs."