Looks like a mistake (and this time I looked it up, 'suit' isn't an adjective). When you google the first part of the sentence verbatim, you get https://books.google.hr/books?id=9Rk7BAAAQBAJ&pg=PT70&lpg=PT70&dq=%22I+met+Holmes+outside+the+address+as+arranged%22&source=bl&ots=oQC-cocD66&sig=1VJ8gqcItQefxrAk7giDVJbQ0KM&hl=en&sa=X&redir_esc=y#v=onepage&q=%22I%20met%20Holmes%20outside%20the%20address%20as%20arranged%22&f=false:
"looking very dapper in his I'm-going-to-a-concert suit and greatcoat"