Some of my favorites that are not extremely difficult, yet continually challenge you in tone, articulation, phrasing, and technique in general:
Jiggy-Joggy
Sellenger's Round (Board book)
Valderrabano's Duos and Sonetos (Silva de Sirenas)
Anonymous works from the Siena MS (from Lyre)
Ronn McFarlane's Scottish Lute collection (Mel Bay)
Leonard Williams
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