I'm working on this index of information in a number of books.  This index is different then a normal index in that it indexes certain categories of things.  For example.  Say I have these  categories: Bears, wolves, fish, and snakes.  Under these categories would be a number of different types. Under bears: brown bears, grizzlies etc.  Now I have these different animals show up perhaps multiple times throughout a number of the books (which will have their own categories)  While all this information is separate (all the snakes are related, all the bears are related, they all belong to these books.  What would be the recommended way to organize these so I can search across all the books but isolate my searches as well. Or, am I overcomplicating this?