A non-formal edition statement is normally found within a sentence in the text of the item.
RDA
Record non-formal edition statements in field 250.
AACR2
Do not record this type of edition statement in field 250. Input it in a 500 note instead.
P. ix
"This edition too began to sell in 1954, but then the above mentioned Janet descended on us and soaked a large number of copies of the book. Happily, though stained with the colour of the red cover, many of the books were not otherwise damaged. By the end of 1966, however, this edition also had become epuise. Here, therefore, is what may be described as the Third Edition. Corrected as far as possible ...?
250 Third edition. -- [RDA]
500 "Third edition"--P. ix. -- [AACR2]