When I was very young, I used to love collecting and solving various types of gamebooks. These were essentially mini-solo-DnD-like adventures along the lines of choose-your-own adventure. It all started with the Fighting Fantasy series; the first book, Warlock of Firetop Mountain, was released way back in 1982, and the series went all the way up to book #59 before stopping in the later half of the 1990s. Although the books were geared towards young adults in terms of the level of writing, they were first and foremost puzzles which could be enjoyed by young and old alike.
There were a number of other series which enjoyed brief popularity, such as Lone Wolf, Way of the Tiger, and my favorite of all time: Blood Sword. The Blood Sword books are extremely rare, with only 5 in the series seeing print. Book 5, the rarest of them all, makes infrequent appearances on ebay, closing at around $200US each time. Fortunately, there is a site that has scanned each of the five books, which could be found here:
http://www.the-underdogs.org/showbook.php?id=26There is also much discussion, fan-made adventures, and reviews of the series that started it all, Fighting Fantasy:
http://www.advancedfightingfantasy.com/Anyone else familiar with these books?