The art of the travel poster

I have a particular fondness for old travel posters, and tend to buy them if I see them whilst travelling (reproduction, not originals!) I particularly love the ones from the 1920s advertising the London Underground. My favourites are those advertising 'going to the sales' by the tube, or the London Zoo series. I saw this in a shop window in London, and it captivated me. Fortunately the shop was closed, as it was VERY expensive.