Rosemary Sutcliff’s The Eagle of the Ninth is rooted in  the history of a real Roman legion. A couple of years back I noted some references about the history from a website that has now disappeared – by one Ross Cowan. He had written that

… to learn more, especially about the evidence for the legion in the period c. AD 118-161, see :

Birley, A. R. The Roman Government of Britain. Oxford: 2005, 228-229.

Birley, E. B. ‘The Fate of the Ninth Legion’ in R. M. Butler (ed.) Soldier and Civilian in Roman Yorkshire. Leicester: 1971, 71-80.

Campbell, D. B. Roman Legionary Fortresses, 27 BC – AD 378. Oxford: 2006, 27-29.

Cowan, R. For the Glory of Rome: A History of Warriors and Warfare. London: 2007, 220-234 and 271-273. 

Eck, W. ‘Zum Ende der legio IX Hispana’, Chiron 2 (1972), 459-462

Haalebos, J. K. ‘Römische Truppen in Nijmegen’, in Y. Le Bohec (ed.), Les légions de Rome sous le Haut-Empire. Lyon: 2000, 465-489.

Keppie, L. ‘The Fate of the Ninth Legion: A Problem for the Eastern Provinces?’ in D. H. French & C. S. Lightfoot (eds) The Eastern Frontier of the Roman Empire. Oxford: 1989, 247-255. Reprinted in Keppie 2000a, 173-181.

Keppie, L. ‘Legio VIIII in Britain: The Beginning and the End’ in R. Brewer (ed.), Roman Legions and their Fortresses. Cardiff & London: 2000, 83-100. Reprinted in Keppie 2000a, 201-218, with addenda at 321.

Keppie, L. Legions and Veterans: Roman Army Papers 1971-2000. Mavors volume 12. Stuttgart: 2000a.

Mor, M. ‘Two Legions – The Same Fate?’ Zeitschrift für Papyrologie und Epigraphik 62 (1986), 267-278.

Sijpetstejn, P. J. ‘Die Legio Nona Hispana in Nimwegen’, Zeitschrift für Papyrologie und Epigraphik 111 (1996), 281-282.