I recently heard that I cant read secular books on shabbos. Is this true and does it apply to newspapers and magazines?
Yes, it is true. Reading anything but Torah is not permitted on Shabbat. This prohibition includes reading newspapers, magazines, weekly business and advertising publications, novels, etc.
In the Shulchan Aruch it discusses that even just reading the caption under a painting is prohibited on Shabbat.
Reading Torah books which are written in English is permitted. Examples would include Jewish books on Basic Judaism, Classic Mussar, Emuna-Faith, Jewish Thought…
Shabbat is a special day of rest and it is sacred.