In the UK most works are protected for the life of the author plus 70 years.
In countries party to the Berne Convention, and in many other countries, the duration of copyright provided for by national law is as a general rule the life of the author plus not less than 50 years after his death.
Sri Lanka is party to WIPO convention and literary works are protected there for life of the author + 50 years.
Pieris, P. E. died in 1959. That means that his works are not in copyrights any longer in Sri Lanka. You may copy substantial parts of the books.
Regards Yelena Shlyuger
ILL, Library School of Oriental and African Studies University of London Thornhaugh Street, Russell Square London WC1H 0XG UK e-mail: firstname.lastname@example.org