The excelent book of Hammond and Scull has this little tidbit of information about this issue.
The excellent book in question is The J.R.R. Tolkien Companion and Guide, vol. 2, Reader's Guide, the article "Reading".
However I'd like to know something that is intriguing me: which was their source to this statement?
Unless we've missed something else in our notes, our source for the statement was an unpublished manuscript comment dated April 1940, in which Tolkien mentions The Sword in the Stone, clearly having read it. (As this is an unpublished, in fact restricted, manuscript in the Bodleian Library, we can say no more about it without permission.) Mr and Mrs Findegil have now been discussing the definition of soon in "soon after its publication", and whether that word should, after all, apply to an act of reading which could have occurred at any time within a span of nineteen months. It could have been immediately after the first publication of The Sword in the Stone in August 1938, or it could have been later; at any rate, it was early enough for Tolkien to have enough appreciation of White's book to be able to make a reasoned comment about it in April 1940. Perhaps, given the pressing events of this period, with ill health and the outbreak of war to distract from recreational reading, "soon" may work in a relative sense.
Since Tolkien took the Times, he may have seen the review of The Sword in the Stone on 2 September 1938, part of which is remarkably like what was said later of The Lord of the Rings: "Mr. White's book is one of those about which it is impossible to be lukewarm: those who like phantasy [sic] will revel in it; those who dislike phantasy will find it quite unreadable." C.S. Lewis, though, who could be said to like fantasy, found The Sword in the Stone "one of the most deeply vulgar books I've ever read" (letter of 11 December 1940).
Which interesting assumptions and thoughts we can have by focusing in the comparison between the two creators?
Richard West long ago (January 1970, in Orcrist 3 = Tolkien Journal whole no. 11) put Tolkien and White together, with C.S. Lewis, in "Contemporary Medieval Authors". And Tom Shippey in J.R.R. Tolkien: Author of the Century included White with Tolkien, and others such as Golding and Orwell, in an interesting discussion of postwar English writers.
the sequels that were , intentionaly or not, a trilogy of books
See Letters, p. 221: The Lord of the Rings "is not of course a 'trilogy'. That and the titles of the volumes was a fudge thought necessary for publication, owing to length and cost." Or see Reader's Guide, pp. 554-5.
Wayne & Christina