These were small, pocket sized hardback books with what seemed like hundred of subjects to choose from. No surprise really as they were produced from 1937 - 2003 (by Frederick Warne & Co in the UK)
The books were about a huge variety of topics from hobbies, art, history and wildlife. The title of the books came because they wanted to be of interest to the "observer" - people who were lookng out and curious about life (hence they were popular amongst children).
Now-a-days most of these books are collector's items. The values of the books can vary from 50 pence to hundreds of pounds. So it might be a good time to go and start rummaging in the loft!
In 1983 Warne was acquired by Penguin books. Warne bought out new editions of the Observer's books that were slightly bigger than the original books, and were in paperback, not hardback.