Hi,
As mentioned, the Osprey books are very good (

) and easy to obtain. Besides "Nieuport Aces of World War I", try:
Osprey Aviation Elite Units No. 17, "Spa 124 Lafayette Escadrille" by Jon Guttman.
Elite Units No. 18, "Groupe de Combat 12 'Les Cigognes", also by Jon Guttman.
Not an Osprey book, but Jon's new book on "Nieuport Flyers of the Lafayette" from Albatros Productions has many superb (and new) Bob Pearson profiles of Nieuports 11, 16, 17, 23, 24 and 24 bis and a really terrific cover by Robert Karr.
Albatros also did two "Specials" on Nieuport Fighters which I believe are still available, and superb. Get 'em.
The book of all books is "Les escadrilles de l'aeronautique militaire francaise, Symbolique et histoire, 1912-1920", published very recently by the SHAA. Very difficult to obtain, but worth it.
Greg