Regnault is invaluable. I just picked up a copy for about $120.00 and it was well-worth it. Combined with the articles I have on the same subject by O. Hollander, I now have a pretty good picture of the eagles and standards of the period. Some think Pierre Charrie's newer study is better, but some think Regnault is the definitive study. Take your pick, probably. One of the photographs in Regnault is the 84th Ligne's 'Un Contra Dix' plaque.
The Elting uniform study is necessary to anyone interested in the uniformology of the period. However, L500 is a little steep. I'd check ABEbooks and Amazon and see what comes available there.
I have Fallou, and it is excellent with much information to be gleaned from it. I would recommend it if you are interested in the Guard.
Those are the ones I have and all are keepers in my opinion.
If you don't want the one on the Gendarmerie, let me know. Pick it up for me or let me know how to get it and I'll send the money. I am very interested in it. If I were you, I'd get it.