Amy Jeff's Storyland (lovely children;s edition now available) was a big hit in hardback my first Christmas back in bookshops, and then a bigger hit in paperback last year. Thomas William's Lost Realms feels like an obvious choice if you want to follow that interest in the early medieval world, it's ticking over at the moment, and I think that by the end of the year it'll prove to have been a considerable hit.
I haven't read my copy yet because it was immediately claimed by my husband - he says it's excellent. It concentrates on the forgotten kingdoms of ancient Britain - the ones '..that hover in the twilight between history and fable, whose stories hum with gods and miracles..' That was enough to sell me on it. Reviews have been excellent as well which is also encouraging, and what little I have read suggests that there's plenty of the sort of vigorous debunking that's always fun to read. It probably doesn't hurt that it's Anglo-Saxon adjacent given my current enthusiasm either.