I have all three books: Zherebukh, Kovalchuk, and Delchev, and though I just started playing the Grunfeld this year and have not read any of the books cover to cover, I have read a lot of all of them and can comment a bit on each.
Zherebukh gives very dynamic lines, and generally I like his recommendations, though the coverage is not as thorough as Kovalchuk, and the book lacks the 'main ideas' section that Delchev has, which I also found nice for understanding typical themes when learning a new opening. In the exchange, his choices seem rather similar to Kovalchuk's, except for in the 8.Rb1 line, where he recommends Qa5+ and taking on a2. Some of the lines are really creative and dynamic, like the Nh5 lines in the Bf4 Grunfeld (as seen in the sample), which I had not seen anywhere else before this book. I also quite like that he chooses a different system than Kovalchuk against the Russian system (Nc6/e5/Nd4 pawn sac), which Delchev covers briefly as a backup line. Against 3.f3, he gives the same line as Delchev, but there are some differences, and I feel his coverage is more thorough (such as what to do against an early ?!0-0-0). He gives the same line as Delchev vs. the Fianchetto, but there are some early differences, and in the Bg5 system with Nf3, he also gives a rather interesting and unusual line. Basically, he gives very dynamic options whenever possible, such as 5.h4 dxc4, instead of 5.h4 c6, and I feel the lines are thematical and in the spirit of the opening, though some are certainly risky. I really need to dig into this book more, though so far I like it.
Kovalchuk is the most thorough, as can be seen when you compare his 10...b6 exchange variation mainline to Zherebukh's. The former will spend an entire section on some variations, whereas the latter will often just give a note and one or two variations. Whether one can memorize all this or not, I don't know, but it is certainly useful for correspondence players. Aganst the fianchetto variation and 5.h4, Kovalchuk gives lines with ...c6, which don't feel in the spirit of the Grunfeld for me, but to be fair, I haven't tried them yet. Also, against 3.f3, he gives 3...c5, leading to a Saemisch Benoni position, which I was a bit dissapointed to see, though I suppose it is objectively safer than 3...d5 and might be nice to have as a backup line. His coverage of the Russian system is excellent, especially when compared to Delchev's, which is lacking, though there are some differences, such as Delchev recommending ...Bb7 in the mainline, and Kovalchuk the more traditional ...Be6. This book is also full of novelties and improvements, often early on in minor lines, and lives up to typical QC standard. I think it is definitely the best of the three books, though as I mentioned, I am not thrilled about some of the recommendations.
Delchev was my first Grunfeld book, and I quite like it, even though it is lacking in some places, such as the Russian system. I like that he has the typical chess stars sections (main ideas, step by step, complete games), and I also like that he often covers two lines for Black, even if a bit superificially. I think one of the best parts of the book is his coverage of the fianchetto variation, which seems much more indepth than Zherebukh's, though I should read the latter more before really making that claim. I also like his anti-Grunfeld section, which not only covers the English and Reti, but also annoying stuff like the Tromp, Barry Attack, etc. Zherebukh has anti-Grunfelds too, but they do not include d-pawn specials like the Tromp, and I cannot find the line 1.c4 Nf6 2.Nc3 d5, as it seems like he only covers Nf3 before Nc3, but maybe I am missing it somewhere.
Overall, I think a serious Grunfelder should hae all three books, as they are all good in their own ways and different enough to complement each other. For big mainlines, like the Exchange and Russian, I think Kovalchuk is best, whereas for sharpening things up, avoiding ...c6 structures, or back up lines, I think the other two books do a great job.
I hope this helps!