Complex sentence: N. J. Higham, who wrote a book on Gildas and the literary tropes he used, has suggested that Gildas may have had considerable motives for placing attention to Ambrosius.
Split into simpler sentences: N. J. Higham wrote a book on Gildas and the literary tropes that he used. He has suggested that Gildas may have had considerable motives for placing attention to Ambrosius.