It was crap when he wrote it. Don't use Oman either. The best stuff is the more modern material in Stephen Morillo's 'Warfare under the Anglo-Norman Kings' and his collection of articles on 'The Battle of Hastings'. Matt Strickland's book on 'Knighthood and Chivalry ' is also good, as is his collection of articles called 'Anglo-Norman Warfare'. I don't know what their US publishers are.
Ospreys are a good substitute - but be warned that they still use old ideas.
The Terry Wise one on 'Anglo-Saxons, Vikings and Normans' is useless, so stay away from it. The Warriors series book on 'The Normans' is quite good, as is the one on 'Warriors of Outremer'. The Elite series on 'The Normans' is a bit dated. The Armies of Charlemagne' give a good overview of the development of early medieval western Frankish armies. The new one on 'Medieval German Armies 1000-1300' give a good spread of piccies and ideas on the development of the armies of the eastern Franks.