The notion of Lanark and Thaw's stories being parts of the same book came from The English Epic and its Background by EMW Tillyard, published in 1954, discovered in Denniston public library. It astonishes me to think there was a time when the non-fiction shelves of libraries in working-class Glasgow districts had recently published books of advanced criticism!
Ah, yes - I remember those days. Sadly, the British public library has been in decline since Margaret Thatcher's romantic involvement with the market (continued by T. Blair and G. Brown) and the decline of any feeling in government for responsibility for the 'public sphere'. Once upon a time librarians from the Nordic countries used to visit Britain to see examples of the best in public library systems and services - all the traffic would have to be in the other direction today.