Page 31 has:
> … if, as a result of some error on the part of the programmer, the order Z F does not get overwritten, the machine will stop at once. This could happen if the subroutine were not called in correctly.
> It will be noted that a closed subroutine can be called in from any part of the program, without restriction. In particular, one subroutine can call in another subroutine.
See also the program on page 33.
The Internet Archive has the 1957 edition of the book, so I wasn’t sure if this wording had changed since the 1951 edition. I couldn’t find a paper about EDSAC from 1950ish that’s easily available to read, but [here’s a presentation with many pictures of artefacts from EDSAC’s early years](https://chiphack.org/talks/edsac-part-2.pdf). It has a couple of pages from the 1950 “report on the preparation of programmes for the EDSAC and the use of the library of subroutines” which shows a subroutine listing with a comment saying “call in auxiliary sub-routine”.