Louise Penny hasn't published anything about the writing process, but Elizabeth George has written a book on that topic: Write Away (2004).
The writing books I most often recommend are the two by Stephen King:
Danse Macabre (1982). It's the same material he used for a writing course at University of Maine, "Themes in Supernatural Literature." So you get the actual class he taught--in book form.
On Writing (2000): It's half-memoir about his experiences as a writer, but also a master class for how to write well. Some literary critics consider this book among the finest non-fiction works ever written. It's so good that I actually consider it a character flaw if someone doesn't own a copy of it--and I am *not* much on reading SK's usual output. I adore all of his non-horror works, but his standard fare gives me many a sleepless night for weeks after reading them. Bloody hell, I *still* have nightmares about Pennywise from It. 
The best writing advice, ever, isn't in a book at all, but in the essay, "Politics and the English Language," by George Orwell. You don't even have to buy anything to get it. It's available here:
https://www.orwellfoundation.com/the-orwell-foundation/orwell/essays-and-other-works/politics-and-the-english-language/
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