Books with false centers are often used.

4 seconds sound clip from the Reefer Madness (1936) movie soundboard.

You can hear this line at 00:06:33 in the Blu-ray version of the movie.

Quote context

[...]

- The dried leaves and berries are ground up and made into cigarettes by a simple hand machine.

- The deadly narcotic is that quickly and easily prepared for its market.

- The sale of marihuana is even more difficult to detect and halt than the traffic in drugs such as opium, morphine, and heroin.

- They are hidden in fake jewelry cases, in the heels of shoes, women's shoes especially, because the drugs can be secreted in false heels.

- Hollowed shaving brushes are another medium.

- Books with false centers are often used.

- Watch cases are convenient hiding places.

- The value of drugs thus seized is enormous.

- Recently, a huge supply of heroin was taken. It was concealed in an apparently harmless shipment of 35 barrels of olive oil.

- The deadly drug was burned in the incinerator of the Bureau of Engraving and Printing.

[...]