One of the better introductory book for the Troubles is Belfast Diary; War as a Way of Life by John Conroy. The author, a reporter for the Chicago Tribune, went to Belfast and reported on multiple sides and tries to be as unbiased as possible (and complete neutrality on the Troubles is as common as hens' teeth) and covers multiple aspects of the Troubles from the ground up. Interpreting Northern Ireland by John Whyte is an older text, but still a valuable one. It is a decent survey of the underlying social, political, and economic evolution of Northern Ireland and how they contributed to the Troubles. Whyte's book though preceded the peace process and so is a bit outdated in this regard. Northern Ireland 1921-1996 by Paul Bew, Peter Gibbon an Henry Patterson is an excellent academic survey of the long-term evolution of Northern Ireland from the Anglo-Irish Treaty through to the start of the peace process. One of the benefits of Bew et al.'s book is that they are really strong in tracing the rise and evolution of various political parties, including the DUP which is now strangely relevant to UK politics. For the DUP and other Protestant groups, Susan McKay's book Northern Protestants: An Unsettled People talks about various internal factions and regional groups and how they coped with the evolution of the Troubles.

*aside: a number of Irish specialists both in public and on their own social media circles are actually quite floored by this recent development vis-a-vis the DUP and the Tories. Northern Irish politics have existed in their own cul-de-sac for quite some time and for them to have a national importance in UK politics is just strange to see.