The first Bloomsbury paperback of Chamber of Secrets was published in 1998 with the ISBN 0 7475 3848 4. A first impression has the full number line, 10 9 8 7 6 5 4 3 2 1, on the copyright page. If the line ends at 2 or higher, it is a later printing.
That ISBN is easy to confuse with the hardback's, 0 7475 3849 2, and online listings often mix them up. Check the binding as well as the number: this one is in pictorial card wrappers with Cliff Wright's cover art.
Some copies carry a Smarties Book Prize 1998 sticker on the front. It is a nice touch but does not tell you which printing you have.
Like every early Harry Potter paperback, this one was bought to be read. Spine creases, rolled corners and fading are normal, and an unread copy is genuinely scarce. Grade carefully: the difference between Very Good and Near Fine is worth more here than on most paperbacks.
We are still checking whether this paperback appeared on the same day as the hardback. If it did, dealers would treat it as a first edition in its own right, as they do for Philosopher's Stone.
Sources: the Sotheby's guide to identifying Harry Potter first editions and descriptions by UK specialist dealers. Format, date and ISBN come from Open Library's edition records.