I couldn't answer why Mach number replaced IAS at high altitude given the IAS will always give you a reading of some fashion
Quite correct and you could operate with reference to IAS if you wished. However, that would involve having a cheat sheet alongside so that you could keep track of a constantly changing IAS limit with height .. so, to make life easier for the pilot (and that's the only reason we prefer to use the machmeter at height) we fly Mach rather than IAS.
Alternatively is there a reference book to explain this?
Any of the undergraduate aerodynamics/performance texts (eg Aircraft Performance, Austyn Mair and David Birdsall, Cambridge Aerospace Series) are useful but probably a bit too much into the mathematics for the typical pilot (who really doesn't need the esoteric detail .. unless one derives a masochistic pleasure from that sort of stuff).
For pilot use, probably one of the more useful texts is Hurt's Aerodynamics for Naval Aviators. This has the usual pilot relevant equations for note but is a very easy simplified read on the story. Readily available in just about every technical bookstore flogging aeroplane stuff.
A number of the posts above have referred to temperature - not relevant to Mach.