Here is one of the best, if not the best, stories of mystery and tragedy of the 1900 years. To ''Four-Pools,'' a quiet stock-farm set in the beautiful Shenandoah Valley, comes a worn-out New York lawyer for a vacation. He is a witness of many curious things and of the final tragedy. The unraveling of the mysteries which follow each other with a cumulative interest is due to a clever New York newspaper chap. It''s an unusual and stirring story, in which the portrayal of life on a Southern plantation and the horror and fear roused in the negroes by what they believed manifestations of the ancestral ''ha''nt'' is clearly and vividly done. A book that will keep its readers up nights.