I found it tedious and dull.
Maybe I was too young. Also probably disappointed that it wasn't like James Bond.
Deighton was a little more cerebral than Fleming I think. Try some of his other works.
Len Deighton's protagonists aren't physical heroes, overcoming the bad guys with grit, strength and the ability to endure pain and suffering, and keep going. They are small cogs in a government machine, struggling, sometimes in vain, against the stupidity, passivity, ignorance, fecklessness and parochiality of their superiors.
I probably would not have liked them in high school either.