A large portion of these illustrations were for novels by Irish author Lord Dunsany. Authors influenced by Dunsany include H.P Lovecraft, Robert E. Howard, Clark Ashton Smith, J.R.R. Tolkien among others. The full books can be read at archive.org at the following links..
The Last Book Of Wonder (1916)
The Book Of Wonder, A Chronicle Of Little Adventures At The Edge Of The World. (1912)
The Gods Of Pegana (1911)
A Dreamer's Tale (1910)
The Sword Of Welleran, And Other Stories (1908)
Time And The Gods (1906)
"The partnership of illustrator Sidney H. Sime and fantasy writer Lord Dunsany (also poet, dramatist, and grand chess master and pistol champion of Ireland) is without peer in the annals of fantasy illustration. It is almost inconceivable to imagine a Dunsany story - with its exquisite fusion of elements from Greek and Celtic myths (Dunsany was friendly with Yeats and the writers of the Celtic Twilight), Arabian Nights adventure, and the solemn harmonies of the Old Testament - without the drawings of Sidney H. Sime. Sime has been called the "greatest imaginative artist since William Blake," and aside from their fin-de-siecle elegance, and delicacy of line recalling Persian miniatures, Sime's drawings manifest that rare faculty of being able to give definitive, and often uncanny, form to the poet's merest suggestions." - quote source
Images found at Golden Age Comic Book Stories Pic-ectomy and other places I've lost track of.
If anyone has more scans from Bogey Beasts, please share them with me at evilenergy @ yahoo dot com Thanks!