This is a reference manual from the early 1990s about using some of the parts from an XT-class computer to less expensively build a more modern 386 or 486 PC. It appeared in the July, 1994 Micro Systems catalog for $14.95 (plus four dollars shipping and handling).

According to the catalog, the book provides the example of turning a Tandy 1000 SX (an 8088) into a 40 MHz 386 at a cost of under four-hundred dollars, at the time. It would probably still be possible to accomplish this, but the availability of components would make it more difficult.

Note: Although the above-mentioned example isn't very possible or desirable today, the general concept remains valid; some parts are interchangeable.