Then, while browsing through some old tournament games I came across the game below and noticed how far my opponent and I managed to get into the book lines in the French Winawer even though I had never read a book on it...can't speak for my opponent.
Shortly after that I read an article by GM Greg Serper in which he stated his deep belief that the best way to learn openings is to analyze good games played by great players. This way not only will you improve your general level of chess, but also learn specific opening ideas.
Bent Larsen said in his book of selected games when describing one of his biggest chess achievements (Amsterdam 1964) that while most of the participants were preparing by researching the latest novelties of GM Boleslavsky (who was one of the best theoreticians of that time), he was studying games of Greco and Philidor!
A lot of amateur players get too caught up in opening theory and it harms their general development. Don't buy a repertoire book about a specific opening, but get one that gives an overview of everything with the general ideas. Then play over master games with the openings you want to play. That way you'll get a sense for the middlegames and endings that are likely to occur.
One of the opening I used to play was the French, Winawer Variation (because that was one of Botvinnik's favorites and his One Hundred Selected Games was one of my favorite books. The following old OTB game from back in my tournament playing days shows what can happen. The first 13 moves showed up in my (expanded) Fritz 12 opening book. What that let's me know is that even if you've never studied a book on a particular opening, if you play over enough games with it, something will stick and you'll be able to play it fairly well even without having tried to memorize lines. Your opponent probably hasn't studied the same lines so won't know the best moves anyway. In that case, as soon as he leaves the book you're on your own.
I have told the anecdote before about the time I played a young opponent who was blitzing out his opening moves almost instantly while I was using much more time. Eventually, somewhere around move 15 or so, he went into a long think and I knew we we at the end of his book knowledge. It only took him another couple of moves before he made a game losing blunder. In the post mortem he was spouting analysis and when we got to the point where I had deviated from his memorized line, he was very critical of the move I had played. When I asked him why it was bad all he could say was it wasn't what Bobby Fischer had played in that position. My reply was that if it was so bad why couldn't he refute it? No answer.
Recommended for further consideration:
IM Greg Shahade on Opening Books