Thank you all for your suggestions.
@Alias: I didn't know about Soviet chess 1917-1991, I will definitely take a look into it.
@Smyslov_Fan: No, it wasn't due to special endgame work, quite to the contrary

I didn't know about Shekhtman's book on Petrosian, I will check it out when I have a chance.
@TonyRo: I'm already a proud owner of Larsen's best games, I enjoyed the book very much.
@Katar: I have some of Kasparov's red and blue books. Frankly, I didn't gain much from reading them - the historical accounts are mostly bollocks IMHO and the game commentary is too uneven for my taste.
@ReneDescartes: I didn't know about Botvinnik's books either, but will check them out.
The challenge now is to buy the next chess book only after I finished the previous one. I'm still working on Yusupov's training course, but once I'm done with it I will look into Botvinnik's books next, I suppose.
Thanks again for all your suggestions.