Widor was a great organist and teacher. During church services, he used to have improvisation contests with his assistant, a young Gabriel Fauré. His book was created as a supplement to the by-then outdated Berlioz Grand Treatise above. It’s useful mostly for its outlook and its charts of different techniques, like trills, tremolos, double-stops, and maximum tempos of pizzicato etc. Some of these values have changed, but most are essentially the same. Always check with a modern manual before following his advice, though.