why the author chose the title"the river between"

Ngugi wa Thiong'o chose this title because it is relevant to the book from whatever angle you chose to look at the events in the book.

In the book,there is a river called Honia that divides but at the same time connects the two ridges; Kameno and Makuyu.This river as one can deduce from the many references to its usefulness,is quite important to the community.This may be the superficial source of the title.Looking at it from a deeper perspective, a river is a source of livelihood to the people who live near it whether it is from fishing or any other activity.  Therfore the river in this case could symbolise land and culture. This is mainly what brings about conflict between the Europeans and the Africans.Land and culture(mainly Religion) are some of the strongest concepts of an African Community .By bringing a different culture through introduction of Christianity and taking African land,Europeans contaminated the 'purity' of the Gikuyu tribe,just as a river would become polluted by introducing substances to it that don't belong.

  Another reason I can think of is that the title fitted the geographical setting of Ngugi's novel.