An item that had captured my attention is the stamp book issued for King Juan Carlos I's 60th. birthday, Edifil 3544C. The listing in the Edifil catalogue is somewhat vague.

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The stamp book contains the then current 100, 200, 300, 500, and 1,000 pesetas high value permanent stamps of King Juan Carlos I. The first three appear se-tenant with a label depicting the King's Arms in a numbered miniature sheet (Edifil 3544). The latter two appear se-tenant with two labels showing the Spanish Arms and the King's Arms in a numbered miniature sheet (Edifil 3544).
According to my Edifil catalogue (2023), 40,000 stamp books were printed. This is the same number given for the print run of the two miniature sheets. Although I have not found confirmation, I have found indications the stamp book is the only source for these miniature sheets.
The booklet also contains four pages with numbered proofs of the 100, 200, 300, and 500 pesetas stamps printed in black (Edifil 64A/D). The numbers on the two miniature sheet pages and four proofs are the same and also appear on the back cover of the stamp book.

Edifil mentions that the stamp books were sold for 10,000 pesetas. They contain stamps to a face value of 2,100 pesetas. At the official conversion rate, the price equals € 60. This is quite an expensive item with a hefty implied surcharge for the four official proofs.
In 2023, Edifil priced the stamp book at € 150, the two miniature sheets at € 50, each, and the four proofs at € 50 the set. I bought mine at auction at the starting price of € 30, half the original price. Of course, the auction house charges a fee of 20%. I, already, had to pay the service fee for postage on another item, so at € 36, I could satisfy my curiosity. ...