The Glasgow firm Blackie and Son enabled a dream collaboration of E. K. Chambers as editor and Talwin Morris as graphic desinger to create the Red-Letter series (1904-09).
Hathi-trust Library webpage with links to the first 25 volumes. (There seems to be 38: a search for TNK suggests that Sir Edmund omitted it.)
The small volumes (16 x 9.5 cm) were bound in ivory cloth with black and red lettering and design inside and out. The tissue-paper covers were either tan or red. To find a full set in pristine condition, wrappers intact, is rare. Library catalogues often assign volume numbers, but these appear to be spurious.
Morris (1865-1911) was an adherent of the Glasgow School of Art Nouveau. He designed book series covers for Blackie and Son. He portrays himself in one of his own capitals (T). Blackie chose him to be the designer of the set.
The somewhat otherworldly natural designs incorporating buds, blossoms, and vines reflected the Art Nouveau movement. And the life-forces inherent in the graphics complement those at work in Shakespeare's living text.
Morris puckishly signed his work in morse code with three dots and pauses that spelled his initials.
Pinterest page devoted to his book designs
Morris bore no relation to William, the colleague of Dante Gabriel Rossetti, but the latter was certainly an influence.
Sir Edmund K. Chambers needs no introduction to those who study Shakespeare and other Elizabethan drama. He was a founding member of the Malone Society and wrote the longtime standards The Mediaeval Stage (1903), The Elizabethan Stage (1923), and William Shakespeare: A Study of Facts and Problems (1930).