Charles Keeping
Here are a few illustrations by Charles Keeping from a book called Horned Helmet, written by Henry Treece and published in 1963. I had never heard of Keeping, but unlike many of the older illustrators I talk about here, there is a good amount of info about him online. It appears he had quite a career, working as a reluctant cartoonist, illustrator of all the requisite classics (Beowulf, Dickens, Frankenstein, etc.) as well as creating several award-winning children's picture-books.
I like what I've seen of his stuff quite a bit. So many book illustrators obviously, slavishly, use photo reference (if not outright tracing), resulting in turgid, by-the-numbers "Reader's Digest Condensed Book" type work, but Keeping's figures are intuitively proportioned, his compositions fluid and organic, and his line work - my favorite aspect of his style - aggressively expressive. His drawings look like they were made from skeins of black wax unspooled across the page.
I've made several of these scans larger than usual (you'll have to scroll, unfortunately) but figured it was worth it to see the quality of line, texture and detail in these examples.
Detail of a two-page spread.
Detail of the above image.
Detail of the above image.
Four bonus images from Keeping. First (left) is this full-page sample from his picture-book The Christmas Story, from 1969.
Next up is this cute color piece from his 1982 interpretation of the Rudyard Kipling story The Beginning of the Armadilloes. Awww!
These two final images are from Margaret J. Miller's Knights, Beasts and Wonders, published in 1969.