Complex sentence: All these books are colourful and entertaining, and contain some valuable information about Britain's imperial wars in India, Sudan and South Africa, but they are essentially exercises in self-promotion, since Churchill was already a Parliamentary candidate in 1900.
Split into simpler sentences: All these books are colourful and entertaining, and contain some valuable information about Britain's imperial wars in India, Sudan and South Africa. The works on the latter contain elements of in self-promotion, since Churchill was already a Parliamentary candidate in 1900.