Where Adler's book is the 'coffee table' book, McDowell's - 'A Study of Colt Conversions and Other Percussion Revolvers' - is the 'bible' for serious collectors.
It's expensive, as you'll note should you look for a copy for purchase - because of demand and because there was a single printing before McDowell died, and that greatly affects pricing on the secondary market.
You should be able to borrow a copy from your Public Library by using an Inter-Library Loan.
Vaya,
Scouts Out!