Complex sentence: He then began an association with the Leicester Gallery and Hodder & Stoughton; the gallery commissioned paintings from Dulac which they sold, while publishing rights to the paintings were taken up by Hodder & Stoughton for reproduction in illustrated gift books, publishing one book a year.
Split into simpler sentences: He then began an association with the Leicester Gallery and Hodder & Stoughton; the gallery commissioned paintings from Dulac which they sold. The rights to the paintings were purchased by Hodder & Stoughton, who used them as illustrations in illustrated books, publishing one book a year.