It

From Author Wars - https://authorwars.com/publications/it-571691.html

Overview:

Title: It
Author: William Mayne
Year: 1980
ISBN-10: 0140311742
Publisher: Puffin
Pages: 191
Binding: Paperback
Type: Novel
Title Reference: It
Notes:
Data from Amazon UK as of 2016-05-24 (no list price) -- date 1981-02-26 -- 18 cm OCLC: -- date 1980 (c) 1977 -- 19 cm

Date and size should be checked. As of 2016-05-24 two of four booksellers at ABEbooks give 1980 (one 1981, one silent); one gives precisely 180mm (three silent). No one at ABEbooks shows a 1st ed. or 1st US ed. cover image.