fsgbooks:

From the New York Public Library:

In his copy of Franz Kafka’s “The Metamorphosis,” Nabokov retranslated virtually the entire work interlinearly; on this page, he has depicted the beetle into which Gregor Samsa metamorphoses.

There’s reading a book, and then there’s Nabokov reading a book. “Interlinearly” means he translated the book between each line. For the entire work.