Well, it's as I expected in one way: Leavis is very judgmental and allows little room for argument. The first half-sentence affirms that "[t]he great English novelists are Jane Austen, George Eliot, Henry James and Joseph Conrad", and the rest of the book is an elaboration of the greatness of the latter three (Jane Austen having received a separate book of her own). Not having read much of the authors in question, let alone of those who Leavis dismisses as less than great, I can only really react by assessing whether or not Leavis gives me a fresh understanding of those books that I have in fact read, and also by taking his recommendations of books I haven't read as potential future reading.
Leavis does not really satisfy me on the first count. His concept of "greatness" is nowhere clearly enough defined for me to feel whether or not I agree with it, let alone whether or not it's a useful criterion for assessing the quality of a novel. We all know that there are good books and bad books, and most of us will agree that, say, Pride and Prejudice is good, Jonathan Livingston Seagull is bad, and American Gods is good but flawed. Not everyone will do so: there are plenty of people who find Austen's prose impenetrable, Bach deep and meaningful, or Gaiman either indigestible or worthy of uncritical admiration. It is sometimes nice to imagine that there are vaguely objective criteria out there which one can appeal to, and I had sort of hoped that Leavis would fairly clearly signpost what those criteria might be. But he doesn't.
However, if I take Leavis' analysis as an expression of taste, his taste is sufficiently close to mine (we diverge on Wuthering Heights, where I know that I am in the minority who find the book pretty unappealing, but are agreed on Middlemarch and Heart of Darkness) that I did find his recommendations of other novels worth reading, including several by writers outside his chosen few, very interesting: the following therefore go on my Bookmooch list and my Amazon (hawk, spit) wishlist:
Benjamin Disraeli: Coningsby, Sybil and Tancred
George Eliot: Adam Bede and Daniel Deronda (though Leavis recommends skipping the bits that are actually about Daniel Deronda and concentrating on the bits about Gwendolen)
Henry James: The Portrait of a Lady and The Bostonians
Joseph Conrad: Nostromo and The Secret Agent
Charles Dickens: Hard Times
A final thought: I'm writing this on a train and won't edit again before I post when I reach my hotel this evening, but I'd be very interested to know if Heart of Darkness might have influenced H.P. Lovecraft. (Or, if I have the chronology wrong, vice versa.) Leavis entirely fairly accuses Conrad of going well over the top, in the style of a 'magazine writer' influenced by Kipling and Poe rather than with the subtlety he was capable of. But the passages he chooses to illustrate this point seemed to me very reminiscent of At the Mountains of Madness. I guess that probably (as Leavis sort of implies) the two have common roots in pulp literature.