edited by Harper & Row, $15.00. Granted that every anthology contains too much of X (reader’s choice) and not enough of Y (ditto), Mr. Nims has made a superb collection of English poetry from 1200 to the present. His selections are generous, well chosen, and sometimes surprising. There are short biographies of the authors, lavish explanatory notes accompanying the poems, and an appendix explaining the terms and techniques of the art. In his introduction, Mr. Nims points out that reading poetry should be fun, and he has certainly done all an editor can to make it so. Additional notes, index..