The best book on gliding for beginners I´ve ever seen, is the ´Manuel du pilote de vol à voile´ from the french aeroclub (which is in french language, unfortunately)
Auteur : SFACT. 240 p. Format : 21 x 29,7 cm. 10 phases de progression. Questionnaire. Carnet de progression. Carte radio nav. Echantillon. Le tout
présenté dans une pochette. 33.00 €
If you have a technical background, this might be a very interesting read :
Fundamentals of Sailplane Design
by Fred Thomas
Foreword by L. M. M. Boermans (President, OSTIV)
ISBN 0-9669553-0-7
Library of Congress Catalog Card Number: 99-61150
9" x 10.5", 286 pages, hardcover
Contents
Basic Aerodynamics and Flight Mechanics: Fluid dynamics and the sailplane - Airfoil and wing theory - Performance and handling qualities - Aeroelasticity
Design Requirements: Certification requirements - JAR-22 - Cross country theory - Thermal models and climb performance
Sailplane Design Optimization: Basic considerations - Wing design - Variable geometry - Flying wings - The World Class - Fuselage and wing-fuselage transition - Empennage and controls
Determination of Performance and Handling Characteristics: Theoretical methods - Experimental methods
Trends in Sailplane Development: Sailplane class definitions - Early glider designs - Sailplane development to date - Outlook
Appendices: Tabulated design data and three-view drawings - "Zacher" flight test protocol - German-English glossary of technical terms