Jack Vance is another author you could enjoy if you like Gene Wolfe. Vance is known for his vivid and strange worlds, as well as his complex characters. His 'Dying Earth' series has that same sense of otherworldliness that you can find in Wolfe's novels. The language and the settings in both authors' works are quite rich and engaging.
China Miéville could be an interesting choice too. His books, like 'Perdido Street Station', are full of strange creatures and unique urban landscapes. Similar to Wolfe, Miéville creates really complex and detailed fictional universes that require some mental effort to fully explore, but it's well worth it for the rich reading experience.
Eric Ambler is another author. His novels often deal with the world of espionage and intrigue during the mid - 20th century. The political and social contexts in his books are somewhat similar to what Furst presents in his novels.
You might like Steve Berry. His novels are also full of historical mysteries and adventures. He often takes real - life historical events and adds a fictional, thrilling twist to them, much like Dan Brown does.
The Nero Wolfe series is another option. Wolfe is an eccentric detective who solves crimes from his armchair, while his assistant Archie Goodwin does the legwork. The novels are filled with interesting characters and intricate mysteries, much like the Spenser novels. And the relationship between Wolfe and Goodwin has some similarities to the relationships in Spenser's world.
I'd recommend starting with 'The Book of the New Sun'. It's a classic of Wolfe's work and gives a great introduction to his complex and imaginative writing style.
The Hercule Poirot series could also be appealing. Poirot is a very different kind of detective from Pendergast, but the mysteries are top - notch. Agatha Christie's writing style is so clever, and Poirot uses his 'little grey cells' to figure out the most convoluted of cases. It has a more cozy mystery feel in some ways compared to the often darker Pendergast novels.
You might like Lee Child's short story collections related to Jack Reacher. They give you more insights into his character in bite - sized stories. Also, books by Vince Flynn featuring Mitch Rapp could be appealing as they also have a tough, resourceful protagonist dealing with dangerous situations.
You might like books by Robert B. Parker. His Spenser novels have a similar tone of tough - guy detectives solving crimes in an urban setting. The main character, Spenser, is also very charming and has a complex set of relationships, much like Stone Barrington.
James Rollins is a good one. His novels, like Steve Berry's, often blend adventure and mystery with historical elements. For example, his Sigma Force series has exciting plots that involve ancient secrets and modern - day threats.
One of the best Gene Wolfe novels is 'The Book of the New Sun'. It's a complex and beautifully written science - fantasy epic that takes readers on a journey through a far - future Earth. Wolfe's use of language and his ability to create a vivid and detailed world are truly remarkable in this work.
You might like 'Smile' by Raina Telgemeier. It has a similar feel with great characters and a relatable story about growing up.