Any books which make my imagination roam and wonder..... So even the Holy books to me are a bit science fiction.... But lacking a lot of the science.
Since you did not define books. Transformers,micronauts,rom,fantasic four,shogun warriors,uncanny x-men,peter parker the amazing spiderman,silver surfer,what if,robotech,i could go on but i'm watching tv.
Star Trek for one.
Any book on science that give me the feeling I'm being drawn into reading nonsense.
Books on global warming.
Star Wars Books.. X Men.. Dr Who Books.. Alien Books..Tron E T..
Dan Brown book's,and book's you get out of the science fiction section of the library.
Science fiction, to me, is any book that has gadgets or ideas or characters that are not based on today's reality or upon history. Science fiction allows your mind to wander into the realms of the unknown. I would suspect science fiction is harder to develop into a book than say, a book based on a historical happening. It would require much more imagination.
Books which I consider to be *good* science fiction are those which take one currently untrue scientific principle and make it happen, e.g. Faster than light travel, but then make all the other science internally consistent. IOW, suppose we had FTL travel, what else would that cause to happen in engineering and in society?
The other day I was watching the orginal Batman (1966). They had all kinds of gadgets and technologies that were not realized at the time. For example they had a crime fighting computer in the Batmobile to help them solve crimes. At the time this was pure science fiction but like many things in science fiction they were later realized. Today computers are standard equipment in many police cars. So the Batman comic books were science fiction. I consider a book to be science fiction if it has technologies in it that have not been realized when it is written.
I don't really bother to figure out the genre of books I read.
A book in the future, or another world.