I have long been a fan of Christian Zacharias, having bought his first recording of the Schubert G major Sonata D894 on LP [and I still prefer it to his later remake], and his first LP of 11 Scarlatti sonatas, recorded in 1979. Two further LPs followed, each with 11 sonatas, recorded respectively in 1981 and 1984. All 3 were issued as a box set in 1985 [which I have long cherished]: but, as John Duarte noted in 'Gramophone' at the time, there were several mistakes in the documentation.
The contents of those 3 LPs occupy the first 2 CDs in this set [76'55 & 73'39]. The third [70'32] consists of a group of 16 sonatas recorded in 1994, issued only on CD. The fourth disc here [a big mistake, in my view] consists of 20 different live performances of the sonata in G major Kk.55, given as a regular encore at recitals all over Europe between 1973 and 1994 [63'12]. The individual timings vary between 1'55 and 3'24, and the recordings range from good to barely adequate. It is a sonata which does not otherwise appear in the set, and it would surely have been better just to have picked the best one and added it to the third disc.
The 4th CD aside, this is a highly desirable set, well recorded and played by the pianist who, in my opinion, has as much to say about Scarlatti as any since Horowitz. It is offered at bargain price, but the notes are only in French.
Added in Edit: Notwithstanding my reservations about the 4th CD in this set, I note that used copies of the original single CD issue are now commanding astronomical prices online, presumably because it sold very poorly and is thus extremely rare. One cannot, unfortunately, assume this will enhance the resale value of the present set after it is deleted.