Typical cliché advice, which would be useful if they wanted to gain more general knowledge and improve their vocabulary or ortography (and still, for knowledge only educational books would be okay., But I would say that what this person wants instead of just knowledge is to get their IQ higher (Using the knowledge the right way instead of the knowledge per se)

I have seen everywhere that people always recommend books without any hesitation, but how do you expect someone to improve their intelligence reading fictional non-educational books such as Harry Potter or Fifty Shades?

So the answer is, sadly intelligence cant be improved. If you want to get knowledge, read educational books, watch docummentaries, or follow some fun-fact channels on YT. If not, try eating benefitial foods for the brain or consider nootropics (even though these options are not very useful, it would only help you while growing up or preventing things). Apart from that there is not much more you can do, sadly.