Who can identify the (British?) illustrator/artist with initials CS
who painted the watercolour wash design for the dustjacket of the 1915
Shilling Edition of Sir Arthur Quiller-Couch's "Hocken and
Hocken" (William Blackwood and Sons, Edinburgh and London, 1915),
which was apparently sold off from the Blackwood's archive in
the 1980s?

These initials appear in the bottom right hand corner as a simple C
with the top and smaller loop of the S coiled inside the C, the larger
bottom loop mirroring the curve of the opposite facing C. A pair of
dots, appearing as a colon, is placed beside the two top
right-pointing tips of each interlooped letter.

The image itself is of two gents [the eponymous Captains Hocken and
Hunken] in a festive snug with a ruby-red decanter and tumbler type
glasses on a small table between them; each man is holding a long clay
pipe. To the left there appears a moonlit harbour scene [of Fowey]
with a square-rigger and twinkling night lights. The mention '1/-net'
in large red characters is placed inside a white filled circle at bottom
left, and the book title, author's initial ("Q") and name (Sir A.T.
Quiller-Couch - this last in brackets)  are placed across the top. The
image measures 35.5cm (height) x  23.5cm (width).

A nicely worded question, but the book title is incorrect :) It should
be "Hocken and Hunken". I haven't found the answer to your question
but have a query out to a knowledgeable source so we'll keep fingers
crossed.

You are of course quite right, and as you will have seen from the
later mention of the two main protagonists, I did know this. A moment
of inattention is my only excuse!

Unfortunately, my email queries came up dry and I've been unable to
run down anything definitive on the Internet. However, Amazon.com
carries this title which sounds interesting: "Dictionary of British
Book Illustrators and Caricaturists, 1800-1914 With Introductory
Chapters on the Rise and Progress of Art: With Introductory chapters
on the rise and progress of the art" by Simon Houfe. I suggest
checking your local library (the book retails for $60.00, but I found
it in the on-line card catalog of my local library). Here's the Amazon
link: http://www.amazon.com/exec/obidos/ASIN/0902028731/002-4811431-9504804

Best of luck, 
ericynot-ga

the best professional advice about antiques that I have ever seen is
on the BBC Antiques Roadshow.

Go to : http://www.bbc.co.uk/antiques/antiquesroadshow/

and I think you will be able to get a reply there!
Good Luck