Oh wow, fascinating to read about the Helen Forrester books! I knew about her and her books from Virginia Nicolson's...whichever one of her women's history books is the WWII one, I think it's Millions Like Us, and her story is compelling enough told second-hand; the real thing sounds like a lot.
Me too for being unnerved by the Weirdstone of Brisingamen! I think you have to read those at about the age of 9 to be overwhelmed by them just right. (That said, I'm still not sure I'd read the chapter where Susan gets possessed (?) if I were alone in the house at night.)