This isn’t a way to learn Dewey, but it may help your organization. If you have any books that have been published fairly recently, look on the page opposite the title page. At least, that’s where it is in most books. Many times there is cataloging information there, including Dewey numbers.
For example, here it is for B is for Big Ben by Pamela Duncan Edwards. It’s actually on the back of the title page. Summary is given; ISBN; suggestions for subject listings in the library catalog, then the number 941, the Dewey number for the British Isles.
For those that don’t have this information, called tracings, BTW, you could probably use the overview you found (World Book has a good one) and put them in general categories.
Does that help?
Karen