There are no books that are directly similar, because Lee Child is the top thriller writer currently writing, with a unique style and approach to story. While there have been many writers who have tried the "invincible loner hero" thing, after you've read the Reacher books you are bound to be disappointed in almost everything else. (That said, as of 2023 Lee Child has retired from writing the Reacher series and turned the franchise over to his brother, Andrew, who is trying hard but simply isn’t as good a writer).
That said, to my mind the three (plus one) closest sets of books—all really well written—would be:
- Stephen Hunter's series starring Bob Lee Swagger (as well as the prequels and sequels featuring other related characters)
- David Baldacci's series starring both John Puller and Will Robie (although most of his other series, including the Camel Club, King & Maxwell, etc. are also excellent)
- Robert Crais's series starring both Elvis Cole and and Joe Pike.
- The “plus one” is a new series from up and coming author Nick Petrie about his character Peter Ash. While there are only three books so far, even Lee Child has said “Lots of characters get compared to my own Jack Reacher, but Peter Ash is the real deal.” I agree.
Other series to sample to see if they are to your taste would be:
- Mark Dawson about John Milton
- Daniel Silva about Gabriel Allon
- Jack Higgins about Sean Dillon
- Vince Flynn about Mitch Rapp
- Joseph Finder about Nick Heller
- Zoë Sharp about Charlie Fox
- Stuart Woods about Stone Barrington
- David Duffy about Turbo Vlost
- David Morrell about John Rambo
- Sean Black about Ryan Lock
- Patrick Lee about Sam Dryden
- Stan R. Mitchell about Bobby Ferguson
- Don Winslow about Neal Carey
- Brad Thor about Scot Harvath
- Andrew Gross about Ty Hauck
- Mark Dawson about John Milton
- Matt Cook about Austin Hardy
- Jeremy Robinson about Jack Sigler
- Michael Connelly about Harry Bosch
- Robert Bidinotto about Dylan Hunter
- Andrew Peterson about Nathan McBride
- Richard Bard about Jake Bronson
- James Abel about Joe Rush
- Dennis Lehane about Patrick Kenzie
- Jack Coughlin about Kyle Swanson
- Anything by John Grisham
- Anything by Nelson DeMille
and of course the classic, best selling series:
- Tom Clancy about Jack Ryan (Senior and Junior)
- John D. MacDonald about Travis McGee
- Robert B. Parker about Spenser (as well as his other series)
- Robert Ludlum about Jason Bourne (as well as his other books)
- Clive Cussler about Dirk Pitt (as well as his other series)
- Steig Larsson (and his successors) about Lisbeth Salander and Mikael Blomkvist
and if you want to go really faaar back, like, say, the 1960s and before, try:
- Lesley Charteris about Simon Templar (The Saint)
- Ian Fleming about James Bond
- Fran Striker about The Lone Ranger
- Anything by Helen MacInnes
- Anything by Alistair Maclean
(Did I mention that I read waaaaaaaay too much crap literature? Or that I've read every book in every series mentioned above?)