My father recently moved to a new home and turned up a stack of D&D-related items he hadn't seen in years. He doesn't have an interest in D&D anymore, not for a long time, and I don't know anybody who does, so I'm hoping the value of these items will be somewhat better than them sitting on a shelf for another generation. The items are all Advanced Dungeons & Dragons, 2nd Edition, and are as follows. Please tell me if there's any missing information;

2101, PLAYER'S HANDBOOK
2117, THE COMPLETE PSIONICS HANDBOOK
2110, THE COMPLETE FIGHTER'S HANDBOOK
2128, MONSTER MYTHOLOGY

There's also two maps, from Vengeance of Alphaks (9148) and The Wrath of Olympus (9189). Maps only.

To clarify, I have no idea what I'm talking about. I'd appreciate some help here, please and thank you.  

The items you list are not worth very much, but their value is dependent on their condition.  If they are in mint condition you might get $20.00 for the player's handbook and $10.00 for the other items if you are lucky.  Usually, these sell for about half that, or less, on eBay.  

eBay is one of the best ways to determine an item's value IF you look at what these items actually sold for and not what other people are selling them for.  As you look at what various items are selling for, you can look on the left-hand side of the page and see where you can check a box in the "Show Only" section entitled, "Sold Listings."  Once you click on that, only sold listings within the past 90 days will be displayed.  Then you can observe how items in better condition sell better than those in poor condition and at what price.  Personally, I recommend listing the three guidebooks in one listing and the Player's Handbook in another listing on eBay.  You'll get more money that way than trying to sell them at a local comic book and gaming store.  Just make sure you take good photos of the items and describe them accurately.