Probably you might like to put these items on your Someday/Maybe list or sublists if you wish. IMHO it's a matter of the guide author's preference, not the GTD guideline.
Perhaps the author was thinking of a physical file of physical, paper-based information. Rather than duplicate the information on a paper or digital list, the thought was to throw it in a folder. Of course, in my life today, the notion of a paper file of things to buy seems quaint. Even the weekly drugstore circular arrives by email. Of course, some people still use paper quite a bit, but I’m not one of them.
I think your solution is best for me.Things to read, movies to see, and similar lists can be both someday/maybe and reference. Your someday/maybe list could include items that you want to review on a regular basis. You can also just collect items on lists to consult on an as-needed basis. An example: I have a "Gift Ideas" list that I store as reference. I don't keep it as a Someday/Maybe because I don't review it regularly, but I will consult it when there's an upcoming birthday or other occasion.
as other users have said, it is both; technically, someday/maybe is a special category of reference list. Early on in the GTD journey a lot of people have only one someday/maybe list. As you go along using GTD, its common to build up hundreds or even thousands of someday/maybe items. That's when breaking out multiple someday/maybe lists generally becomes useful. The examples given in the guide are simply more specific categories of someday/maybe lists.