I found the endgame very frustrating. Managed the grid fill fairly quickly but could not identify the quotation despite having all the words except the title and the author. Googled it every way I could think of.
After 2 days of intermittent tinkering I recognised the end of a longer word in 17, then 18 and it dawned I needed to add letters to resolve ambiguities.
So I completed the puzzle but was still none the wiser about the work.
I now know the source from an earlier post in this forum but the only way I can get it from Google is to put the whole quotation including the title in the search engine with quotation marks around.
Borderline unfair for those of us who are not familiar with this author.