Joshua Wong made a second public appearance since the controversial new Chinese security law came into effect.
Critics say it erodes freedoms guaranteed in Hong Kong's mini constitution, the Basic Law.
He's pleaded not guilty to three charges including inciting, organising and taking part in an unlawful assembly.
Wong's fellow activist Agnes Chow, who stood next to him while he spoke to media and was accused of inciting and taking part in the same unauthorised assembly, admitted the charges against her.
Over the weekend, a search for books by Wong or pro-democracy politician Tanya Chan on the public libraries website showed the books, including "Unfree Speech," co-authored by Wong, either unavailable or under review.
A government department said on Sunday (July 5) books are being reviewed to see whether they violate the new law.