One of the best ever is the classic W.H.B. Smith title
'Small Arms Of The World - A Basic Manual Of Military Small Arms ' ( Military Service Publlishing, Pennsylvania ). Originally published in 1943 and many subsequent reprints.
It's a brilliant book, detailing every WWII smallarm of all the main combatant nations, complete with cutaway views and field-stripping instructions for each gun.
It's a big old book but you should be able to find a copy on Bookfinder or suchlike without too much trouble. I got mine in a secondhand shop for £6 and that was a real bargain....
