There's a neat, short autobiographical essay by Sergey Shipov (which doubles in passing as an ad for his outstanding books on the Hedgehog) that can be obtained from Mongoose Press. He discusses his career as a player, then as Garry Kasparov's sparring partner in blitz, and then in a non-playing capacity. He concludes with some highlights from his career, focusing on a 2006 win over Magnus Carlsen.
You can look through the goodies for yourself (the booklet/essay can be obtained as a free PDF by writing mongoosepress [you know the symbol to use] gmail.com), but I will focus on just one of the things he wrote. I've heard from when I was a kid all the way up through a year or so ago that Kasparov was mainly an openings expert. A very strong grandmaster all-around, obviously, but he wouldn't have been such a dominant force aside from that particular strength. Even Hikaru Nakamura has made a comment to that effect. By contrast, here's what Shipov had to say on the matter.
By the way, those fools who for years explained Kasparov’s dominance only by his opening superiority (which, let me point out, is not a gift that falls from heaven, but rather comes from hard labor) simply had no idea what they were talking about. I remember we played six games of Fischerandom chess, and there was no battle there at all! In completely unfamiliar positions, Kasparov’s advantage over me was far greater than in normal chess. In the absence of the usual pathfi nders his flights of fancy, his sense of dynamics, and his ability to instantly separate the important from the secondary became particularly salient.