Plane and Spherical Trigonometry

The classic treatise on spherical trigonometry, covering spherical triangles, Napier's rules of circular parts, the solution of oblique triangles and applications to astronomy and geodesy. A clean digital edition with numerous examples.

Isaac Todhunter

A rigorous classic that treats plane and spherical trigonometry in depth, with careful proofs, spherical triangle solving and applications to astronomy and navigation.

William Chauvenet

Short, clear notes that derive the fundamental formulas of spherical triangles from plane trigonometry, including the spherical laws of sines and cosines. A quick modern entry point before the classics.

Rob Johnson