This small project is more about design rather than the (relatively simple) electronics.
This seems like an ordinary bookcase, but on closer inspection one book is the odd one out. It's a "book" with a built-in temperature display.
The fake book contains an integrated thermo-/hydrometer. The "book" itself is made from perspex with a cover glued on.
Hidden between the real books it doesn't stand out too much, but I have found out that once people have discovered it they can't take their eyes off it!
If people are interested I will gladly write a description of the electronics (PIC with Sensirion sensor).