This is a big question, but I'll take a stab at giving you an overview.

The label "accidental" for the most part stems from the fact that the British never exactly had a concrete plan for acquiring the territories that would eventually become their empire. Places like India or the 13 North American colonies (excluding Georgia; a penal colony) were settled by private ventures. Such ventures would have been religious groups like the Puritans or business efforts like the East India Company. These specifically are colonization efforts, in which British groups settled and governed the area in question. Nobody ever sat down and said "OK, we will settle here, here, and here next." These ventures, once successful became part of the "Empire" by virtue of the fact that British subjects lived there, and there was a certain obligation to protect and care for them (in addition to the fat profits their colonies made).

However, a lot of colonies were, as you say "informal", in that they were not explicitly British colonies, but sort of fell under their influence nevertheless. By the 18th century, the British had changed their tune on policies concerning their Empire. The Empire had grown very large, and was proving to be difficult and expensive to maintain. More emphasis was placed on commerce. Why govern and police a colony when you can just enjoy the benefits of its' trade? The United States serves as a rough-n'-ready example here. After the American Revolution, U.S. trade with Britain resumed fairly quickly. The British could enjoy the fruits of trade with North America in both goods and money without the burden of administrating that area.

Emphasis on trade did not mean that the British would no longer acquire new territories; they now chose based not on suitability for the settlement of British subjects, but on potential profitability. The Cape of Good hope and Egypt after the construction of the Suez Canal, for example were both valuable positions on the trade routes to the Far East. Even ships of other nations would stop there and pay the British handsomely to do so. African interests were motivated mostly by the valuable resources that could be found there. Chinese trade posts were established to get a slice of the tea trade. Let me point you to The Tools of Empire by Daniel R. Headrick for some of the hows and whys of the above.

The case of Egypt is particularly interesting, Britain bankrolled the Khedeve, which was allowed to govern on its own as long as trade continued.

But even trade interests could draw the British into involvement, and they would flex some muscle if a local power disrupted the flow of goods. The Royal Navy would sail in, smash up the place and leave in a "bash and bolt" expedition. Such expeditions were far easier on the British than full time management.

India is the obvious exception here, but it was too valuable to give up. That in itself displays some of the case-by-case policy making at work in the Empire.

I'm afraid I can't recommend anything online to you. Don't let that stop you from looking though, I'm sure its out there!