Etymology of "catch a bosso"
Solution 1:
Green's Dictionary of Slang has boss, "a view, or sight, of", which he suggests is derived from boss-eye, a "squinting or injured eye, or person who has one".
This is said to be derived from boss, "to make a mess of, to spoil".
The sources quoted for this are all in the 1880's, with an example from 1926, which fits quite well with your book.