I'm a great admirer of Codreanu and make no apologies for it. As you rightly say, he wasn't Catholic but he is an example to us all. His ideas inspired many, who were searching, to become Catholic.
'The Prison Notes' by Codreanu is obligatory reading as far as I'm concerned. Reconquista Press republished it in English in recent times.
'For My Legionaries' is also recommended. 'The Nest Leader's Manual' also is excellent. Another book that is excellent is 'The History of the Legionary Movement' by Horia Sima.