www.alphadictionary.com

This seemingly simple English word has a distinguished past - and present. It has an ancient PIE ancestor and many, many cousins, uncles and other relatives in the Indo European family, including Russian, Czech, Lithuanian, Polish, German and Dutch. Yet it is not much used today.

Surely it is more frequent than that. While not an everyday word, still I hear/read/use it now and then.

Here in the hinterlands, delve is just about extinct. Sometimes one hears it in "dig and delve" which is obviously redundant. I think the seven dwarfs delved for a living. In the figurative sense I can see a use for it as an occasional substitute for "research".

I would say it is somewhat used here. For example, I might say, presented with a knotty problem, I'm not going to delve into that bag of snakes until I have finished project X.