FOR some nine years past, the Astronomical Society of the Pacific (Merchant's Exchange Building, San Francisco, California, U.S.A.) has issuedat first irregularly, then bi-monthly, and later, in response to growing demand, once every montha series of leaflets explaining, in simple but authoritative terms, astronomical matters of general interest. These leaflets have become widely known and appreciated, not only by the general astronomically-minded public for whom they were primarily designed and whose interests have been given first consideration through out, but also by working astronomers as a convenient and trustworthy source of information on certain facts to which reference is frequently required. For example, in Leaflet 30, Dr. F. C. Leonard, of the University of California, gives an account of the newly-discovered planet, Pluto, with the most accurate data available at the time and a useful diagram showing the relation of the orbit to the orbits of the other planets of the solar system; while the distance-velocity relation characterising the extra-galactic nebulae is explained with a table, diagram and photograph in Leaflet 37 by Mr. Humason himself. The first fifty of these leaflets, each bearing the date of writing, have now been bound together and re-issued as a small volume, and it is satisfactory to note that, since the monthly publica tion of the leaflets is to be continued, further volumes may be expected in due time. The collection, which it is impossible to praise too highly, unfortunately bears no indication of its cost, but we feel confident in saying that no one interested in astronomy, what ever his degree of knowledge of the subject, will regret the purchase of a copy.