One of the unsung side benefits of researching a nonfiction book is the stuff you learn that never makes it into the finished product. Reading one of C.S. Lewis' letters inspired me to revamp my note-taking, specifically the way I mark up books. I know some people regard writing in books as a sacrilege, but it's unavoidable in my profession. Much of the time the books are advance reader's copies (ARCs), unproofed printings for the convenience of journalists and booksellers and not "real" books to begin with, if that's any consolation.
Most critics are moderately obsessed with methods for taking notes on their reading. At minimum, you need a way to flag important points and the passages you may want to quote, as well as to jot down any thoughts of your own.
My former colleague Dwight Garner (now one of the New York Times' daily reviewers) showed me the method I used for years. Hesitant to deface the book itself, he'd flag passages with a tiny dot in the margin. You wouldn't notice these dots unless you were actually reading the book, as opposed to leafing through it, but you can find them when you're looking for them. He'd write down his own notes (and sometimes quote passages) with page citations on a steno pad that he kept with the book. Steno pads are about the same size as a hardcover book, which makes them easier to tote around with it than a letter- or legal-sized pad.
I used this method for years, and as a result had a huge stack of scribbled-up steno pads. Not sure why I saved them, beyond my reluctance to toss the result of so much work, and the fact that they did contain a lot of information. Eventually, I threw them out because they weren't at all useful. It was impossible to find the notes for any particular book among all the pads, and even then they were hard to decipher after an interval of even a month or two.
I adapted my new method after reading Lewis' description in a 1932 letter of how he "indexed" a book, in this case, a work of French history:
To enjoy a book like that thoroughly I find I have to treat it as a sort of hobby and set about it seriously. I begin by making a map on one of the end-leafs: then I put in a genealogical tree or two. Then I put a running headline at the top of each page: finally I index at the end all the passages I have for any reason underlined. I often wonder -- considering how people enjoy themselves developing photos or making scrap-books -- why so few people make a hobby of their reading in this way. Many an otherwise dull book which I had to read have I enjoyed in this way, with a fine-nibbed pen in my hand: one is making something all the time and a book so read acquires the charm of a toy without losing that of a book.
It's so typical of Lewis not to recognize that for most people photographs and scrapbooks -- mementos of their own lived experience -- are categorically different from books they'd read! For him, reading a book was at least as vivid an experience as a seaside holiday or birthday party.
I've adapted this method extensively. I underline passages, then in the margins I flag them with symbols indicating which category they fall into: major points, likely quotations, useful facts (dates, ages, places, etc.), and anecdotes (any colorful fact or detail that can be dropped in to make the review more lively). Other readers -- such as students -- who have other needs might, of course, want to use different categories. Along the top of the page, as a sort of "running head," I will sum up in a sentence or two what that page has to say. (In a biography, for example the running head "Tormented at boarding school" would top a page describing what the poor guy suffered in that environment.) If there's no room for my own thoughts at the top of the page, I jot them down on the endpaper.
This method has several advantages. First, it keeps my notes and the book itself together. Second, it's much easier to skim the book as a refresher or when looking for specific facts or passages later, while I'm writing. (This is even easier if I use color-coded highlighters, though I'm usually not that thorough). Third, the notes amount to an improvised outline of the book. If I'm feeling ambitious and/or want to preserve a kind of digital index of the book for possible future use, I can transcribe them to a text file on my computer, which can be added to a searchable collection of documents. Direct quotes can also be added to my personal file of interesting/inspiring quotations.
True, the book is ruined for other readers after this, but if I intend to keep it, in some ways it's even more useful to me. I can almost instantly find passages that I dimly recall reading years earlier. Doing this sometimes makes me feel selfish and profligate, because I do wind up discarding most of the books I review. (I live in a small apartment.) I try to remind myself that there's a long history of writers' marginalia and no one loved books more than C.S. Lewis did. If he could reconcile his conscience to it, so can I.