I think the monthly all-in 99p sweep may have run its course, but you’ll still get occasional updates. This month the estimable imprint S.F. Masterworks has a big chunk of classics on at 99p.
A Scanner Darkly – Philip K Dick
Inverted World – Christopher Priest
Doomsday Book – Connie Willis
Roadside Picnic – Arkady Strugatsky
Mockingbird – Walter Tevis
The Forever War – Joe Haldeman
Rendezvous With Rama – Arthur C. Clarke
The Dispossessed – Ursula K LeGuin
plus in other editions
Neuromancer – William S Gibson
Altered Carbon – Richard Morgan
Revelation Space – Alistair Reynolds
Rendezvous With Rama and Revelation Space are both exceptional novels in the genre, but they also start long series. The subsequent novels in the Rama series are deeply, deeply disappointing and by novels 3 and 4 barely readable (Clarke clearly by this stage in his career farmed out the writing to the considerably less talented Gentry Lee. The revelation space series is a complex and rewarding universe, but nothing quite packs the punch of the first. I’ve not read the sequels to the Forever War but again its an incredibly influential and readable novel.
And Hogfather by Terry Pratchett.
Some real classics in that list