The author of a current historical novel uses past tense in most of its 38 chapters, and present tense in five or six. The novel is a New York times bestseller: The Women in the Castle. Can someone explain the possible rationale?
I have done a cursory inspection through the book, and I discovered that these chapters were told in, as you say, present tense:
• 9, 26, 27, 28, and 29 in the first group;
• 34 ⸨but only first few sections thereof⸩, 35, 36, 37, and 38 in the second group.
The first group are those which occur entirely in the memories or thoughts of certain persons in the story. Not all details are recounted in present tense, but they are told as events immediate to the perspective of the narrator. One advantage to the different narration is a layer of distinction from other sections of the text.
The second group are in the conclusions to the stories and take place in 1991. There, it seems to me that the shift in tense is to place the reader in a position of impending occurance — like the instantaneous rate–of–change in differential calculus, — and which so tells of things happening now and continuing to happen rather than already happened.
That seems to me to be the gist of it. I could review my other thoughts on the style of narration and such in those and in other chapters, but I don't think that necessary to answer your query.
I haven't read that book, but past tense is usually used for narration and present tense for dialogue. Perhaps the past tense is used for a retelling of the past, and the present tense is employed for an ongoing scene. For example, if the character went to a therapist and imagined his past, then took a break for a chapter to live the current life, then went back to the therapist and continued the past story, and back and forth. Read the book, and you'll probably find out why.