Fallou L. Le Garde Imperiale - At £45 it is a snip - how much detail do you want?!
Les Aigles Impériales et le drapeau tricolore 1804 1815 Jean Regnault - It is a lot of money but it has been out of print for years and £70 is fairly typical for a copy in good condition. It is entirely in monochrome and only covers the Imperial period. Charrié's Drapeaux et Etendards is cheaper, in full colour, covers the same ground as Regnault on the Imperial period, but in more detail, and covers the revolutionary period too - LCV in Paris had copies of Charrié. In an ideal world get them both.
Eltings complete uniform set £500 - I sold mine on for this sort of price about two years ago. The first two volumes tends to attract a higher price. Buy them if you must. I collect military plates as a side hobby which is what attracted me to them in the first place. The difficulty with them is Herbert Knötel's provenance, which is varied and often opaque. A lot of it can be traced to his father's stuff, Forthoffer, Ottenfeld and other secondary stuff. The second two volumes are a particular problem in this context and, generally, they are almost worthless as a reference. My advice is to invest the £500 in some good military plates, Rigondaud, Rousselot, Richard Knötel etc
Fraser is indeed superficial and a bit of a 'yarn'. I can't remember what I paid for mine but it was a facsimile reprint from Worley Publications several years ago.