I'm currently reading up Gildas' account of the Saxon migration, and am struggling to find any specialist books on foederati (Gildas' account essentially saying the first Saxons came to Britain to serve as mercenaries). I would very much appreciate it if I could be pointed in the right direction for some reading on the topic!
In Kingdoms of the Empire : the integration of barbarians in late Antiquity, edited W. Pohl (Leiden Brill, 1997), there is a discussion by Heather about foederati in the 4th century. There are also a few more items that might be of interest in that volume. And from there, the bibliographies (especially to Heather's piece) will get you to further specialist works.
I'm sure this book is debilitatingly expensive. Ask your local library/institution if an inter-library loan is possible.