The Glen Park branch of the SF public library and the ingleside branch are both within walking distance from Sunnyside. There's also a Little Free Library in front of a house near the corner of Congo and Joost : )
Technically I think there aren't any libraries in Sunnyside, San Francisco. The closest ones would be either the library on teh campus of City College just to the south (though it is for the students there) or a little farther south on Ocean Blvd. by Westwood Highlands.