Foullkes is a mixed bag - most of his information regarding armour itself is very dated (check out his section on "maille" as an example). That said, there is some great useful information not readily found elsewhere - most notably in the apendices. He prints in full a rare document on the arming process in the 15th c. (and it is not the one most commonly seen in print, although it has a similar title). He has a useful list of armourers and where & when they practised their art, and also a useful list of armourers marks. As a rule though, check out any information on actual armour itself against better & more recent works on the subject.

Hope this helps.