for a quick overview of a topic
to develop vocabulary or keywords for searching
for suggestions for further sources
Oxford Dictionary of Astronomy, 3rd edition by 
Collins Dictionary of Astronomy, 5th edition by 
A Companion to Astronomy and Astrophysics by 
Patrick Moore's Data Book of Astronomy by 
Encyclopedia of the Solar System
Biographical Encyclopedia of Astronomers by 
The Astronomy Encyclopedia
Encyclopedia of Astronomy and Astrophysics
Astronomical Almanac by 
Physically Speaking by
A collection of quotations pertaining to physics and astronomy. It lets you discover what others have said on topics ranging from anti-matter to x-rays.
The Unified Astronomy Thesaurus (UAT) is an open, interoperable and community-supported thesaurus which unifies the existing divergent and isolated Astronomy & Astrophysics thesauri into a single high-quality, freely-available open thesaurus formalizing astronomical concepts and their inter-relationships. The UAT builds upon the existing IAU Thesaurus with major contributions from the Astronomy portions of the thesauri developed by the Institute of Physics Publishing and the American Institute of Physics. We expect that the Unified Astronomy Thesaurus will be further enhanced and updated through a collaborative effort involving broad community participation.
Cambridge Illustrated Dictionary of Astronomy by 
