The same spelling isn't always pronounced the same way. The letter t in motion, patient, potential sounds like sh. But in words like nature, question, righteous, it sounds like the ch in church.
The letter s in compulsion and tension sounds like sh, but in closure, Polynesia, vision, it sounds like the letter j in French.
Some words change their spelling when -ion is added. Letter d can become s or ss. So divide becomes division, and proceed becomes procession. Others are harder to predict: distinguish becomes distinction, and recognise becomes recognition.
The sh, j, ch and French j sounds described above are called palatals. This book teaches the principles of when these palatals occur and how they are pronounced. Where there are principles behind spelling changes, it teaches them too. A reference section at the end teaches those spelling changes which must simply be memorised.