@Dapplegrey- I think they're reasonably popular; ADTTMOT is available in most bookshops, I think, I certainly see it around. There's also a companion book called "Invitation to the Dance" by Hilary Spurling which is good for keeping track of the plot. I bought that after I'd read the novels though! I read them at 24-25 and loved them. I'm 28 now and would happily re-read in a couple of years so definitely try again!
Oh and Flambards forever!!!! What did you think of book 4 though? I love it, but I read 3 & 4 together and I never thought she should marry Dick, I always held out for Mark. And I cried at the end of book 2 when Sandy died! I'm a bit torn on reading the reboot by someone else, I know it has KMPs blessings but it isn't necessarily how she'd have continued the story for the original characters iyswim? I always wanted to read about Tom and Isobel in ww2, they'd have been about the right ages to be involved. Isobel in the WAAF or a WREN and Tom probably going one better on Mark and Will by joining the parachute regiment or something equally terrifying!