Literary London?

A number of the London Walks discuss literary sights. In particular, their walk in Bloomsbury discusses a number of the authors who resided in the neighborhood:
https://www.walks.com/our-walks/literary-london-walking-tour/

Other literary-themed walks include:
https://www.walks.com/our-walks/shakespeares-dickens-london-the-old-city/
https://www.walks.com/our-walks/in-the-footsteps-of-sherlock-holmes/
https://www.walks.com/our-walks/t-s-eliot-the-waste-land-tour/

I’m sure there are others.

Charles Dickens Museum, British Library, Sathoranos Rare Books, Wallace Collection, Hatchards, Fosters Books; https://www.writerspath.co.uk/routes

Second the above recommendation for London Walks TS Eliot Wasteland or Shakespeare, or anything else led by Rick Jones, who is a wonderful erudite guide. Also Dr. Johnson’s house.

Karl Marx wrote Das Kapital in London, I believe spending time in The British Museum researching and reading.

https://www.marxwalks.com/

You may want to look for London's Blue Plaques, which mark where important people lived, worked, or stayed: https://www.english-heritage.org.uk/visit/blue-plaques/ . Scroll down on the page I linked, choose "Literature" under the Category drop-down menu, and see if any of the names appeal to you.

Visit the authors room in Westminster Abbey where many of the great British authors are buried and with many memorial plaques. A docent can help point out specific ones to you, like the too small one for Jane Austen.

I’ve been to the Keats House in Hampstead which I enjoyed (house and neighborhood, the high street and heath nearby)

These are all good suggestions- if interested in Marx as suggested above, you can also visit the Marx Library which I recommend: https://www.marx-memorial-library.org.uk/

And if in the Bloomsbury/Fitzrovia area to visit some of the literary sites, I recommend heading over to Marylebone High Street to visit Daunt Books, one of the nicest bookshops in London- they have a few branches but this is the best one.