on delivering this book

This clause is also deceptive. In English, there are 12 different meanings to the word “on.” The natural interpretation is that “on” means “at the time of.”

However, another possibility is that it means “having as a topic,” as in “he wrote an essay on the failures of the government.”

If that were the case then the whole of the sentence in square parentheses means that the page of advice was written in readiness about the delivery of the book which is itself “already” prepared for publication.

Etymologically, “already” means, c. 1300, “in a state of readiness” (adjectival), literally “fully ready, quite prepared,” a contraction of all + ready. The adverbial meaning of “by this time, previous to some specified time,” is late 14C.