I've recently read Marcher by Chris Beckett.
Marcher is a stunning novel of alternative reality science fiction from Arthur C Clarke Award and Edge Hill Prize winning author Chris Beckett.
Published for the first time in the UK this, the NewCon Press edition, represents the author’s preferred text, extensively revised and rewritten from the book’s original release in 2009.
Charles Bowen is an immigration officer with a difference: the migrants he deals with don’t come from other countries but from other universes. Known as shifters, they materialize from parallel timelines, bringing with them a mysterious drug called slip which breaks down the boundary between what is and what might have been, and offers the desperate and the dispossessed the tantalizing possibility of escape.
Summoned to investigate a case at the Thurston Meadows Social Inclusion Zone, Bowen struggles to keep track of his place in the world and to uphold the values of the system he has fought so long to maintain…
“With its twin themes of boundaries and mirrors, and the dizzying effect of these multiple views of the same events, Marcher… reflect better than any other SF work extant the true complexity of alternate universes.” – Suite 101