The fourth and most difficult level of reading is syntopical reading.
It is the most complex and systematic type of reading of all. It makes very heavy demands on the reader, even if the materials he is reading are themselves relatively easy and unsophisticated. Another name for this level might be comparative reading. When reading syntopically, the reader reads many books, not just one, and places them in relation to one another and to a subject about which they all revolve. But mere comparison of texts is not enough. … With the help of the books read, the syntopical reader is able to construct an analysis of the subject that may not be in any of the books. It is obvious, therefore, that syntopical reading is the most active and effortful kind of reading.
The fourth and most difficult level of reading is syntopical reading.
https://fs.blog/levels-of-reading/