If it's just one or two you want to add, just add them to the list as is.
If there are enough that it's going to double the size of the section, just add a Readers Section for the moment... probably using the ## notation
... current contents
... the readers you want to add
If it gets unwieldy, I'll split it off at some point. Me and ジョン picked those categories more-or-less arbitrarily, but I'd like to see how big it gets before actually splitting it up.
Might I suggest a criteria for inclusion in that list, however? To qualify as a 'reader', the book needs to be written as one. In other words, the "Parallel Text" books (designed for foreigners) qualify, as do the "はじめての文学" books. But books that are simply "lower level", or "easier for foreigners to read" should probably not be included... or we'll be including everyone's first translation conquest on this list. :-)