Before even picking up a PHB, I bought The Complete Book of Elves.
Though I didn't know until recently, this book has a bad reputation. It's considered overpowered by many. I just grabbed it because I thought elves were cool. The creator actually released a video addressing issues with the book not to long ago. I agree with his points and I still love the book. It introduced me to bladesingers and avariels.
Unfortunately I don't own this one anymore. Several years ago I gave away all of my 2nd material. While it has made moving easier, I regret this decision.