While ridge flying dominated the early days of soaring history, nowadays by far the most commonly used source of lift are thermals.

Every textbook on soaring covers thermal soaring in some detail.  Perhaps the best and most comprehensive explanation comes from G Dale in Volume 1 and Volume 3 of his trilogy “The Soaring Engine”.

G also gave an excellent 2-part talk on the subject that I recommend watching to anyone looking to get more out of thermal lift: