SPOKANE, Wash. — A wise angler will shut up and listen to anyone with 30 years of steelhead fishing experience. In the case of Dan Magers, steelheaders should go further and devote a few evenings to serious study of his new book. Magers, of Big Fork, Mont., has backed his steelheading savvy with detailed records of Idaho’s Clearwater River. The information will boost the probability of catching steelhead in varying conditions in any river using any method, he said. Magers, co-founder of Idaho Steelhead & Salmon Unlimited, has been rubbing elbows with experts on steelhead biology, politics and fishing for …