My husband is steadily working his way through the Patrick O'Brian series, which is about a naval captain in 19th century Britain. He not only uses Britishisms, he uses 19th century naval Britishisms, and 19th century grammar and speech mannerisms. He explains none of them.

The first book is apparently trial by fire - hubby was googling several times per page. If you get past that - apparently enriching your vocabulary greatly in the process, the series becomes quite enjoyable. Hubby says he likes it precisely b/c O'Briens writes in this age-authentic style, keeps his readers on their toes, and never patronizes them. Total immersion in that era.

For myself, I tried reading it, but didn't particularly like the style. It wasn't the Britishisms, though.