"The illustrations of Through The Looking-Glass that appear in this book, finished in 1946, have remained unpublished since then [until Inky Parrot Press’ edition in 2001] to the disappointment of the author, who considered them to be one of her best works for children’s illustration.
Franciszka Themerson commissioned them for George G. Harrap. In 1946, Carroll’s work was about to enter the public domain and the proposals to republish Alice came one after the other. Although printing was concluded near the end of 1947, Harrap decided to postpone the publication due to the ‘difficult market situation’. Franciszka Themerson wished to anticipate correcting the prints to have them ready by the time the publication of the book was decided, but no one gave her any certainty that this would happen, and in the end the issue was left up in the air. When in 1969, twenty two years later, she wanted to recover her drawings with the intention of publishing them through another publishing house, she was told they were unable to locate them. After a year of complicated negotiations, her lawyers (Tarlo, Lyons & Aukin) managed to make them appear and be returned to her." (Source)
And more here.