A lot of passé simple, but that is in every French book except the very modern ones, it's just in schools that they keep telling you not to use it anymore. And a lot of subjunctive imperfect, which would be tricky to most readers, including native speakers, but I suppose not so much to you since you've already read a bunch of older writers who use it too anyway.

I randomly opened the (last) book and came across this sentence which is a good example: "Il ne sortit de son abattement que pour hurler. Hurler qu'on lui sellât céans son destrier, hurler qu'on rassemblât l'ost".

The funny part there is that words that would be difficult for a young or not well-read French reader, such as "ost", are easy for an English speaker who can easily recognize more common English words in them ("host", in this case, as in, an army). It's kind of like in Tolkien, where he'll often use old-English obscure words that make a Dutch reader go "huh, old-English is basically just Dutch". Though "céans" would be difficult for everyone alike. And then the two subjunctive imperfects.