Okay, the first three books out of the Mystery Box of Mystery are:
- Dungeons & Dragons Third Edition Player's Handbook (Wizards of the Coast)
- Dungeons & Dragons Third Edition Dungeonmaster's Guide (Wizards of the Coast)
- Tournaments, Fairs, and Taverns (Natural 20 Press)
So, the first two books are two of the three core books for the third edition of D&D, which came out in 2000. Their release marked a huge tectonic shift in the RPG market, both by having the biggest RPG in history radically change their rules, and second by allowing third party publishers create sourcebooks for the game. Suddenly everyone and their brother were getting into the publishing business.
One example of that process is the third book, Tournaments, Fairs, and Taverns which is full of, not surprisingly, tournaments, fairs, and taverns. Most of the book has rules for all different kinds of games (29 in all): card games, ball games, martial games, etc. There is also a section on alcoholic beverages and rules for the impact of consuming them. Oddly enough, the rules don't include the slight Charisma bonus for light intoxication and the plummeting Charisma negative modifier for heavy intoxication. I'm reminded of an episode of WKRP where Venus Flytrap, participating in a PSA about drinking, actually becomes more agile the more hammered he becomes...
There's also suggestions for creating your own taverns and fairs, and some examples of both. The usability of the sourcebook is quite high. You could adapt the games, et al to almost any other fantasy RPG, and with a little tweaking to sci-fi RPG's as well. It is distinctly crunch-heavy, however, and emblematic of the whole 3.X/OGL movement.
While the third book is distinctly a win, the other two are not. Third Edition D&D came out in 2000, causing outrage among some hard-core D&D enthusiasts who decried the loss of the leaner, more streamlined rules of the earlier editions (many of these enthusiasts continue to play some iteration of the earlier rules to this day). But Wizards of the Coast tested the loyalty of those who supported the new rules by releasing a modified ruleset, called 3.5, a mere three years later. The 3.5 rules cleaned up a lot of problems that the intensive playing of the third edition had revealed, enough of a change to make using material from 3.0 not entirely seamless. I know people still play 3.5, but I don't know anyone who plays 3.0.
You can buy a pdf of Tournaments, Fairs, and Taverns now for $9.95 from RPGNow, so I feel like I'm off to a good start in terms of getting my money's worth.