Three books on Mahler, one reprint of an old one and two new studies, have seen the light: Gustav Mahler by Bruno Walter, with a biographical essay by Ernst Křenek, and an introduction by Erik Ryding Dover, 236 pp., $14.95 (paper) Gustav Mahler’s Symphonic Landscapes by Thomas Peattie Cambridge University Press, 220 pp., $99.99 Mahler’s…