The first 4 Borrowers novels
These alone were originally illustrated by Diana Stanley.
Both this 4-novel omnibus and the 1983 5-novel omnibus The Complete Borrowers Stories (see at ISFDB) have also been issued as The Complete Adventures of the Borrowers.
1st ed.
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Library of Congress (LCCN, below) reports:
-- "[699]"; "Various paging" (numbered in four series?)
-- British Library: B66-20785
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Price from New Statesman 1966-11-11 p707, footnote to Penelope Mortimer "Thoughts Concerning Children's Books" (questioning the need for children's novels, unrelated to this publication)