Son of Thomas Munsey Cook (1817-1884) and Rebecca Jane Sizemore-Cook (1816-1861).

Brother of Sgt. Edward H. Cook (1843-1864), John N. Cook (1844-1898), George P. Cook (1847-1906), Elizabeth J. Cook (1850-Deceased), Calvin Cook (1852-Deceased), Mary Marinda "Minnie" Cook (B. 1854), Ann "Annie" Cook (1856-Deceased), Thomas Boyd Cook (1859-1914), Nelly Catherine Cook (1862-Deceased), Jane Cook (Deceased), and Rebecca Cook (Deceased).

Married to Mary Jane Cooper-Cook (1845-1917) on May 13, 1863 in Wyoming County, WV (per WV Marriage License Register, Wyoming County, WV, 1863, Page 10, Line 22, Application Date: May 13, 1863).

Father of Ulysses Grant Cook (1864-1932), Edward Everett Cook (1866-1940), Florence Viola Cook-Bailey (1868-1943), Nellie Lucretia Cook (1869-Deceased), Thomas Austin Cook (1870-1936), Sarah Elizabeth "Lizzie" Cook-Goode (1872-1895), Cora Ann Cooke (1875-Deceased), Emily Rebecca Cook (1877-1877), John Jay T. Cook (1879-Deceased), Rev. Chester Herndon Cook (1880-1934), William Cassius Cook Sr. (1882-1947), Christopher "Wirt" Cook (1884-1922), and Mary Etta Virginia Cook (1887-1918).

Married to Chloe Page Ellis-Shields-Cook (1864-1942) on October 23, 1917 in Wyoming County, WV (per WV Marriage License Register, Wyoming County, WV, 1917, Page 220, Application Date: October 19, 1917), after the death of Mary Jane Cooper-Cook.

Rev. William Henry Harrison Cook was a State Senator, a Baptist Minister of the Gospel of Jesus Christ, a soldier and a statesman. He died at age 82 from dilation of the heart & senility (per WV Certificate of Death #9109 Dist #5531 Serial #5).

History of West Virginia, Old and New, (1923), Vol. 2, p. 244:
[William Cassius Cook, county superintendent of schools for McDowell County, was born on a farm at Windom, Wyoming County, this state, on the 21st of November, 1882, and is a son of Rev. William H. H. Cook and Mary Jane (Cooper) Cook, the former of whom still resides at Windom, where he was born November 5, 1840, and the latter of whom died in 1918, at the age of seventy-four years.
Rev. William H. H. Cook is a son of Thomas Cook, and the family settled in what is now Wyoming County, West Virginia, shortly after the close of the Revolution, the original American progenitors having come from England and settled in Virginia in the early Colonial period. Rev. William H. H. Cook is a man of fine intellectual ken, he having been largely self-educated, and his life has been one of high ideals and exalted service. As a clergyman of the Missionary Baptist Church he gave pastoral service to four different churches in Wyoming County, and in the early days he frequently rode forty miles in a single day on horseback in making visitation to these churches. He was a gallant soldier of the Union during virtually the entire period of the Civil war, and lived up to the full tension of the conflict. In 1865, shortly after the close of the war, he was one of a numerous company of Union soldiers who marched over the mountains and across the valleys to hold a reunion with Confederate soldiers at Welch, the judicial center of McDowell County having at that time been marked by an open field and a single log cabin. In earlier years Mr. Cook was a successful and popular teacher in the schools of this section of the state, and he has ever striven, with much of ability and fine stewardship, to aid and uplift his fellow men. He served two terms in the Lower House of the State Legislature and two terms in the State Senate. He is a republican, but has worked for political peace and amity rather than for strident partisanship. He has been president of the First National Bank at Pineville from the time of its organization, and in all of the relations of life his influence has been benignant and helpful. Of the thirteen children all are living but one. The seven sons and five daughters have all received liberal educations and all have been successful teachers. The devoted and revered mother was a daughter of Rev. Thomas Austin Cooper, a teacher and a clergyman of the Missionary Baptist Church.]