Once you find a text, my suggestion would be to:
- Read it completely
- Read it again and make consolidation notes under each chapter
- In the chapter consolidation notes, bold any outstanding quotes that you like and would be good in an essay
- Attatch a technique to them
- Make note of the story structure
- Pencil in whenever an author has used subtle or explicit use of genre theory (this just comes down to your research on genre theory in general, and sci-fi as a genre)
- Download 10 past questions and attempt to mould your information to it and see if your values/themes/quotes fit the bill.
Before I recommend a few texts, I IMPLORE you to read and use Dune as your second main text. It is phenomenal, not done frequently or as well. And if you pull it off it works in your favour. Plus it's a fantastic book.
Some great texts would be some sci-fi short stories by prominent authors such as Ray Bradburry, Vast by Linda Nagata, Children of Men ect. Post apocalyptic sci-fis are fantastic.