haggytina:

There is certainly a market for first edition penguin books and they are quite widely collected and wherever there are people collecting books based on their publisher some of them will be very interested in any associated ephemera from the publisher.

The book your offering is quite common and only worth £2/3 in very good condition as a first edition.

I would suspect the sleeve would added fairly significantly to the value as its likely very few have survived in comparison to the the number of books produced in 1956. But without a comparison its impossible to say how much the value is increased.

I would have a look at your listing and change the title to provide more details about sleeve rather than the book, such as including the phrase "penguins come of age" in the title