When Random House began a limited reissue of some of the earlier mysteries in paperback in the late 1970s and early 1980s, the company kept the internal illustrations of the stories but had Stephen Marchesi draw new cover art. Marchesi's work was unique in that his covers were truly cover illustrations instead of illustrated scenes from inside the books. The covers have a light feel to them that convey that an adventure is waiting beginning in Chapter 1; these covers remain as some of the best artwork of the Three Investigators. Below is one example of a cover drawn by Stephen Marchesi.
You can read an interview with Stephen Marchesi at my friend Seth's site at www.threeinvestigatorsbooks.com