Two-box Deep Seeking Metal Detector
by Charles D. Rakes
This project is from Solid State Electronics Projects by Charles Rakes. The transmitter uses a UJT for the oscillator which can be a little hard to find (Mouser and Circuit Specialists have them). This type of oscillator is not particularly necessary and practically any good oscillator design should work.

Rakes also had a later book titled Building Metal Locators which is now out-of-print. It contains many more metal detector circuits but, surprisingly, does not include the two-box design.