I’ve been reading the massive 2,000 page third volume of the Collected Letters of C. S. Lewis (available here). It must be one of the very few books of which I can say, “I’ve only read 700 pages so far”! What an ass the publisher was, not to split it into three.
I’m not finding anything very special in the book. It looks as if it is purely for completists. The volume of letters with a memoir by Warnie is certainly more accessible, and frankly contains nearly all the interesting material.