Wavefronts and rays as characteristics and asymptotics, 3rd editionWavefronts and rays as characteristics and asymptotics, 3rd edition, by Andrej Bona and Michael A. Slawinski, Singapore, World Scientific, 2020, 356 pp., £120(hbk), £95(ebook), ISBN 978-981-122-646-5. Scope: reference. Level: postgraduate, advanced undergraduate