Most "In-Transit" books have been returned to a library other than the one they belong in, and so are en route to the correct library. While listed as "in-transit," patrons may place a hold on the book, so that when library staff checks it in, it will automatically be placed on the hold shelf for you.
If the status of the book remains "in transit" for more than a week, please contact the circulation desk of the owning library to place a search on the book. If it has been in-transit for longer, patrons may place an Interlibrary Loan (ILL) request for the book.