I used to collect a ton of strategy guides when I was younger. I had them for most RPGs I played in the 90's/00's but I sadly got rid of many of them years ago. I still have a few, and my favorite ones have to be my leather bound Lunar 2 and Arc the Lad Collection strategy guides. They had such a premium feel for them back when most guides were just paperback. Plus they're both pretty long games so having a guide for them was extremely helpful!
Back around 2004 my buddies talked me into playing FFXI and my buddy lent me the guide and I remember leafing through it all the time. I picked up my own copy of the guide pretty recently and the nostalgia came back pretty hard when I got it. By all accounts it's a fairly useless guide nowadays, but it holds a lot of meaning to me as it was the first big MMO that I got to play with my friends, and we played for many many years together.