I don't consider myself a collector of anything -- not Blythe dolls (although I have nine Neos and a Middie) or yarn (although I have a stash) or owls (although I have many owls and things with owls on them!) or anything, really. But now that I think about it, I sort of do have a collection... of Sweet Valley Twins books.
Maybe you're familiar with the Sweet Valley High series, or you've at least heard of it! It chronicles the soapy, early-80s-tastic adventures of sixteen-year-old identical twins Elizabeth and Jessica. They look the same but are so totally different, et cetera. Elizabeth is the more sensitive, goody-goody twin who likes reading and writing and always does the right thing. Jessica is the fun-loving, more outrageous yet superficial cheerleader who cares about popularity, clothes, and boys. It's very id/ego, and their interactions and the way they play off each other and how often the fact that they are twins becomes a plot device (switching places, mistaken identities) is all in good fun.
However, as a girl, I missed out on the Sweet Valley High thing; they were before my time, and just too soapy and dramatic for me, even when I was a ten-year-old! Sweet Valley Twins (which takes place during the twins' apparently endless sixth grade year, making them eleven-ish) is what I liked when I was a kid, and still buy from used bookstores occasionally even today. The Twins books are great because it's rare that anything very dramatic happens, but they don't tend to be as cutesy as BSC books. They have just the right amount of cheese for me. So although I have read quite a few High books and some of the other many spin-offs, Twins holds my nostalgic heart.
Lately I've bought a handful more and realized that I have at least 60 Sweet Valley Twins books! Only about a third of them are from my childhood, but in either case, they were purchased second-hand. Others were given to me by friends. I don't even know how I came by some of them, let alone duplicate copies. I haven't even read a few of them!
Here are the titles I picked up today: The Twins Get Caught; Booster Boycott; Big Brother's In Love! (anything with Steven is an automatic buy); Jessica the Nerd; The Charm School Mystery; and The Middle School Gets Married. In my humble opinion, the 40s-70s is prime Twins, so I really scored big. (I also caved to one single Sweet Valley High book: Don't Go Home With John. I hear it's pretty much an epoch if you like the character of Lila. And who doesn't?)