I read "Brideshead" and "The Secret History" so many years ago that my memories of them are too vague for them to have hovered in the background as they have for several Amazon reviewers; so I read "The Lessons" without making comparisons.
First impression: I have read few authors who evoke images quite as vividly as Alderman does. They gripped me right from the Prologue in which she describes a swimming pool full of the half-eaten remains of a meal, thrown in by drunken and stoned party goers. Or there is the memorable description of the huge and dilapidated grounds and mansion tucked away behind a high wall in one of the meaner parts of Oxford. The mansion, with some forty rooms, belongs to Mark Winters, the fabulously rich, gay and Catholic undergraduate, and there he plays host to a group of six fellow-students of both sexes. There are drugs, drinks, and sex. The narrator, James Stieff, lonely and insecure in his first term at Oxford (it is the early 1990s), is part this circle. Mark has them all move out of their college rooms to live, rent-free, in the mansion.
After a while, the novel narrows its focus somewhat to James' two relationships, a loving one with his sensible friend Jess, and the other with the unstable and dangerously unpredictable Mark, though there is, while they were all at Oxford, only one incident which shows how dangerous Mark can be, and his instability is also just about kept under control: there is, after all, the friendship and companionship between them all in that mansion and in that special ambience of Oxford university life, so many aspects of which are brilliantly evoked by the author.
But Oxford life does not got on for ever, and after three years (and a dramatic incident on the last evening which will shape James' life for years thereafter) the little group disperses. James and Jess set up house together, but their contacts with the rest of the group becomes sporadic and marginal - and Mark disappears altogether for the next two years. But then, fatefully, he reappears in their lives. What follows is painful and we know the story is inexorably heading for tragedy and disaster.
I found a significant part of the coda - the device used to tie up loose ends - unbelievable. But this part, too, is compellingly written.