A grown-up dictionary!
I didn't actually work on researching and writing the dictionary, but I was involved in working on the database on which it's compiled and stored; tidying up some technical stuff, making sure everything was structured and formatted correctly, checking that nothing was missing, checking links and cross-references. It was still a great experience though and an insight into a different branch of lexicography. After years of working on ELT dictionaries, where the vocabulary's all very frequent and familiar, and where the grammar's simplified for learners, at first, it was a bit daunting. I learnt loads, having to deal with grammatical distinctions that I'd never really had to think about before and coming across lots of words that were way outside my vocabulary. I may have only joined the project at the tail end, but it was still fun to deal with some 'grown-up' language for a change!