Cirrus@Austlit

'A tiny piece of paper from the year 2019 might not sound very interesting to most people. But Max and Oscar – Bluggsville’s sharpest sleuths – aren’t most people! Max has a hunch that this ancient patch of paper might be valuable, and extremely rare.
Max is right – this isn’t just any old piece of paper. It’s a strange, sticky thing called a postage stamp, and it’s more than 400 years old! It’s an exciting discovery, but before long, it leads Max and Oscar into some very sticky situations…'

Source: Publisher's blurb.