Implement proper Boolean searches
Here are three searches which should produce the same results but they don't. The search results in my library are next to each expression.
(Arius OR arianism) WITHIN 5 WORDS (writing* OR manuscript* OR literature) - 980 results
(Arius WITHIN 5 WORDS (writing* OR manuscript* OR literature)) OR (arianism WITHIN 5 WORDS (writing* OR manuscript* OR literature)) - 698 results
(Arius, arianism) WITHIN 5 WORDS (writing, manuscript, literature) - 114 results.
This gives me real pause about the effectiveness of any search in Logos 6. I'm not sure which search expression gives the correct results.
If Logos is serious about academic and/or professional markets, a proper implementation of Boolean search operators would, I think, be mandatory.

The first two examples were not previously accepted by the Logos search engine. Starting in Logos 6.14, all three ways will be equivalent.