Lone Wolf is a series of gamebooks based in the fantasy realm of Magnamund. They were written in the UK by Joe Dever, who started in the mid-eighties and as far as I can tell, is still writing them. Up to number 20 has been released in the United States, and I understand that there are three more that have been published in Britain. I don't believe there are any plans to publish them here, though.
I read up to #12 as a teenager when they were coming out, loving every minute. (I also read a bit of Fighting Fantasy Gamebooks and GrailQuest and the like as well.) I then spent several years away from them, and recently (around 2005) picked up the rest and am worked my way through them. Yes, they are certainly "young adult" books, but they are still quite fun.
BTW, a "gamebook" is a book you strictly play rather than just read. You read one numbered section, make a decision, and turn to a new section based on that. Most LW books have 300-400 sections, and making it all of the way through will take you to the majority, though not all. So there is some replay value. Since you also have beings trying to kill you in combat, there is a die to roll, and you may end up starting over because a battle was too tough.
Anyway, my point is not to make a completist Lone Wolf site, but to point to relevant resources, and supply one of my own: Action Charts to use in the playing of the books. I never used the ones printed in the books, as I didn't want to deface them, and it would also be inconvenient as well.
- Project Aon, working to bring all of the Lone Wolf books at right (except for the novelizations) on the Internet for free.
- Gamebooks.org, devoted to all gamebooks. (There is a Lone Wolf review page with cover illustrations.)