Once Upon a Time in the East(1993)
(The first book in the Diego series)
A novel by Lionel Fenn (Charles L Grant)
A gunslinger on the run, Diego transports himself from the Old West of the 1880s to 1990s New York to hide and finds that present-day Manhattan can be just as dangerous as the land of tumbleweed.
Used availability for Charles L Grant's Once Upon a Time in the East
November 1993 : USA Mass Market Paperback