Parson, the other problem is at least for Garmins, most of them don't even come with a manual anymore. The manuals are available online, so a lot of people will not go online to look at them. I haven't used them myself, but if you bought a Garmin for instance, they have some online videos to watch
Here. Some of the older models aren't listed here, so if you do a search on Google for instance for Garmin Training Video and then the model such as Oregon 450, you can find other areas as well. There are lots of youtube videos on people walking through GPS features as well, and other websites that have overview videos for specific models.
Besides what has already been suggested, remember simply play with the unit. One thing that scares a lot of people, is they may mess something up. You can always reset the GPS back to factory defaults, so that isn't an issue. The other thing is a lot of people will say, well if I am not hunting, then how can I test it. You can test all Garmin GPS units while sitting in the comfort of your home if you like. What I mean by this is, once you touch the Map screen on a touch screen GPS, or move the cursor on a non-touch screen, you have gone from Live Mode to what I call Browse Mode. Now you can move the GPS around, mark waypoints, etc. It still isn't quite like using it live but you can search for things, etc. It won't allow you to backtrack as it doesn't know where you are but a lot of features will work.
Assuming you have a Garmin GPS, once you figure out your GPS, then you need a NDTRAX map. It will help you in so many ways when hunting ND. The base maps on virtually all GPS units have very minimal information. They are good for marking spots and seeing your tracks, but that is it. I have a demonstration of TRAX maps and many of the demos show ND features
Here
Bottom line is the more you use it, the better you will get at using it. If you only play with it while hunting, it makes it more difficult as when I am hunting, I am concentrating on hunting not figuring out my GPS. Take it with you while doing jobs around the yard, to the grocery store with the wife, etc. Anything you can do to increase your time with the GPS. Also, check with your local sporting goods stores. Some of them have sessions such as GPS 101. The biggest thing with these types of classes and any books, is most will be generic and won't be specific to your exact GPS. They are good for terminology but not for how to use those features on your GPS.