|What makes an index useful?
Look up the word "rock" in a hymnal index. Chances are you can't find it - neither LBW or WOV has even a single listing for it. What about 'shepherd' or 'lamb of God'? No listings in either place either. What is going on?
Most hymnal topical indexes are based on concepts like "repentance" or "grace." That is certainly important, but it's only half the problem. What you need for worship planning is an index of images like 'tree' or 'darkness'. Lutheran Harmony's indexes look not only at the concepts, but also the images, and the people referred to by a hymn. The result is much more useful because Bible passages are often about images. Think of the the 'tree of life' or the 'tree planted by the waters', think of the 'rock of refuge'.
Bible indexes can suffer from the opposite problem. Here hymnals typically index even minor allusions in one verse, but sometimes miss the main point if the language of the hymn doesn't exactly match a verse. In Lutheran Harmony I tried hard to ask myself, what Bible passages would I want to have refer me to this hymn. I hope you find the result useful for worship planning.