Books
OK. For small arms ammo don't bother with anything by Hogg or Weeks. Hogg may be OK on artillery but his small arms writing is very poor.
For British military ammo, either ".303" by Labbett and Mead or "British Small Arms Ammunition 1864-1938" by Labbett.
For German Military 7.9mm, "German 7.9mm Military Ammunition" by Kent or "Die Militarpatronen 7.9mm" by Windisch (in German but with English summaries)
For American, "History of Modern US Military Small Arms Ammunition", Vols I and II, by Hackley, Woodin and Scranton.
General:
"Cartridge Headstamp Guide" - White & Munhall
"Military Rifle and Machine Gun Cartridges" - Jean Huon
"Military Small Arms Ammunition 1945-1980" - Labbett
"Cartridges of the World" - Barnes
Many of the above are now out of print, but can be found from book dealers like Trotman or on Abe Books.
There are lots more small specialised books but the above would be a good start. Beware though, they are often not cheap and books are addictive.
Of course, if you just want to identify .303 rounds, then I will immodestly point you towards my own book on that subject. PM me for details.
Regards
TonyE