A great book for those going to the British capital is The Perfect London Walk by late film critic and author Roger Ebert (https://www.rogerebert.com/roger-ebert/strolling-through-roger-eberts-perfect-london-walk). Published in 1986, the little gem is long out of print, but copies of the 124-page book are available through used book sites (abebooks.com and others) and in used book stores. The work is worth getting for those who seek a "back door" experience in London.
Ebert notes he had taken the walk some 50 times and that it is well known to Londoners. At the same time, the trek is far enough away from the beaten path that most tourists, even sophisticated ones, will not have found it. The walk goes through the Hampstead and Parliament Hill areas of Camden.
Rick Steves fans will love the work in part because it is practical, though it's doubtless some of the information is out of date. Nevertheless, Ebert gives useful information to plan a walk: which Underground lines to take to get to the start, hours to keep in mind for historic sites along the way (the Keats House, Highgate Cemetery, pubs) and suggestions on places to eat (the George, the Spaniards Inn and others). I would check with an up-to-date guidebook on these matters.
The history, culture and experiences along the walk are fascinating, such as Parliament Hill, the tallest place in London. Visitors learn about Queen Boadicea, the Celtic warrior said to be buried on the hill. (As Ebert points out, some believe she is buried under Track Five of Paddington Station.) Walkers can see the cottage where John Keats wrote and wooed Fanny Brawne. Readers learn about the Peasants' Revolt of 1381, celebrated highwayman Dick Turpin and St. John's, Downshire Hill, London's last "proprietary chapel", so called because it is owned by the congregation.
And the walk goes through the Highgate Cemetery, the Victorian resting place of Karl Marx, painter Dante Gabriel Rossetti and author George Eliot. Late travel writer Ian Nairn called the atmospheric Highgate "the creepiest place in London" in his Nairn's London.
The book has dozens of black-and-white photos that are obviously dated but will help guide visitors. The book even has a map, but it is a bit hard to read. I would supplement it with a recently published map.
I recently bought the book having found out about it years ago when I read Ebert's memoir, Life Itself. He adored London, first visiting the city in 1966, and found out about the walk from Daniel Curley, a professor of English literature at the University of Illinois. Life Itself is also great read for avid travelers because of Ebert's boundless love for London.