Does the log say that you found the spell? Under 'A Mysterious Murder' there there should be an entry above 'Evelyn has disappeared...' starting with 'We found a spell...', or it should be in the chest in her house in the corner, near the bookshelf the diary was in.
Do you have older saves to check? if you can find the most recent where you had the spell and the oldest you didn't, that could help narrow down where it may have gone.
It can not be sold and would be removed from a dismissed companion's inventory (unless you put it in a container first) and it can not be crafted with.
The Reveal Spell should show up in the Magical inventory category, and (if you don't recall) looks like a highlighted parchment with a portrait on it.
