I've been reading and re reading A Canticle for Leibowitz since high school and still like it, but find the religion slant sometimes a bit much. Stephen King's The Stand I still enjoy also. The 12 Monkeys, I am Legend, Cloud Atlas, The Road (very depressing) Childhood's End by Arthur C. Clarke, a book called "Dust" where all plant life disappears as the title indicates, that one was really frightening, but can't remember the author....... loads of others too but I can't think of the names just now.... I love stories where everyone dies in the end ! LOL. I've never read The Death of Grass but will definitely do so.
Have just remembered The Book of Dave by Will Self.... I can't write a recommendation as I haven't got around to actually reading it, the dialogue is difficult to read as it is, well, post-apocalyptic and there are lots of arcane words I didn't have the patience to figure out... But I will make another stab at it on holiday.
Swan Song by Robert McCammon. Very, very similar to Stephen King's 'The Stand', but only in subject. End of the world after nuclear attack, the few survivors are slowly banding together...some on the side of good-some on the side of bad, the bad is very, very bad, the good. are very, very....well, you get the idea.
I've read Earth Abides, Flood, the Stand and of course the day of the Triffids. There was quite a good sequel to the day of the triffids by Simon Clarke the horror writer called the Night of the Triffids. Worth a look!