The British Council, Posta Kutusu, Besiktas (tel. 0212/355-5657; www.britishcouncil.org.tr; Tues-Fri 10am-5pm, Sat 9:30am-2:30pm), is a nonprofit cultural outreach center working for educational and cultural relations. The library is one of the best resources for multilingual information on antiquities in Turkey, as well as a great reference center for translations of Turkish literary works. There's a huge collection of English-language books on all subjects, in addition to CDs, music, and videos. The center is popular with students of the English language.
For books and historical documents on Istanbul, the Turkish Touring and Automobile Club runs the Istanbul Library located in one of the old Ottoman houses it restored on Sogukçesme Sokagi in Sultanahmet (tel. 0212/512-5730; Mon-Fri 9am-5pm).
In a forgotten ancient building across from the Fener jetty on the Golden Horn is the Women's Library (Kadin Eserleri Kütüphanesi; tel. 0212/534-9550; Mon-Fri 10:30am-6:30pm; closed for religious holidays). Founded by Füsun Akatli, a renowned Turkish writer, the library has grown into a collection of materials -- mostly in Turkish but some in English -- featuring female artists, photographers, directors, and artisans. There's also a section on women in Istanbul and on women in Ottoman dress.
The Istanbul Archaeology Museum, Osman Hamdi Bey Yokusu, Sultanahmet (tel. 0212/520-7740) has a library of over 60,000 volumes on the subject of archaeology, and will be opened upon request.