QAs the second in the series, 'The Eye in the Door' picks up where 'Regeneration' left off and loses nothing in the way of pacing and style. 
Where the first book focused on mental health among soldiers and war victims and the effects of the trench warfare raging in the fields of France, the second focuses on a quieter war being fought on the home front; that of conspiracies, conscientious objectors to the war and a homosexuality scandal and trial raging across the public sphere. 

What struck me was that this shift in focus never felt undue or from left-of-field. These themes were, in fact, seeded in the first as ancillary problems that were poised to become larger issues in the future so when the second book dives into these issues in earnest, it's actually quite a natural transition. 

'The Eye in the Door' is a much less hopeful book than the first one in the series, I felt, but then perhaps that's on me for expecting a war novel to be hopeful.