Just a thought, but instead of buying another textbook series (which is still organized by lessons with graded grammar, vocabulary and reading passages rather than by topic) as a reference, you might enjoy looking for some supplemental books like a reference grammar or a Korean Grammar in Use (which is sort of a of hybrid specialized grammar textbook series and a reference grammar, rather than a general-purpose textbook, and is REALLY well regarded around here) and a nice K-E dictionary.