Anglers who have lost big fish to weak knots are forever regretful. Better to learn the art of knot tying before casting a line. A new book by Lucy Davidson entitled “40 Knots and How to Tie Them” (hard cover, $16.95, Princeton Architectural Press) explains, in straightforward fashion with easy to understand illustrations, knots most everyone should know — anglers especially. Among these are the blood knot, the perfection loop and the dry fly knot. The 37 other knots in Davidson’s book will come in handy, too.