I’ve acquired a copy of the newly-published book Pillboxes of Britain & Ireland, by Mike Osbourne.
While it doesn’t cover sound mirrors, it is an excellent and comprehensive book on pillbox design and development, with typologies, plans, photographs and details of the theory and use of pillboxen. It is well worth any connoisseur of concrete defensive structures getting hold of a copy.
While we’re at it, I’ll also mention another good recent read, the new Osprey book on Armored Trains. I would review it, but this review says it all pretty well.
Unfortunately the book doesn’t mention the armoured train in Sarajevo.