1.  Robert K. Headley, Jr..  c2003.  Married Well and Often: Marriages of the Northern Neck of Virginia, 1649-1800: Marriages and Marriage references for the Counties of Lancaster, Northumberland, Old Rappahannock, Richmond, and Westmoreland.  Genealogical Publ. Co., Baltimore, MD (online at Ancestry.com):

p.61	BUTLER, the Rev. Amory & CATLETT, Elizabeth (wid.); c. 29 Apr 1672; bride was a dau. of Margaret UNDERWOOD and the wid. of (1) Dr. James TAYLOR, (2) Capt. Francis SLAUGHTER (d. c. 1656/57) and (3) Col. Jn. CATLETT; she apparently had chil. by her second two husbands: Francis SLAUGHTER and Jn., Eliz., Sarah, & Wm. CATLETT; she d. in RapC in 1673 and in her will ment. her dec'd husb. Jn. CATLETT and her four chil. by him, her cousins Wm. UNDERWOOD the Elder, Humph. and Cath. BOOTH, and Capt. Thos. HAWKINS, her sis. PIERCE, her current husb. Amory BUTLER, and her bro. Edw. ROWZIE; this complicated family is discussed in Avant; (RapC RB 1671-76:15; DW 1665-77;136; Avant 3:620; Eaton:23; Slaughter:307; Sweeny: 2-3; Underwood: 91-94)
p. 70	CATLETT, Col. John & SLAUGHTER, Elizabeth (wid.); bef. 5 Jan 1663/64; groom was born in Sitt. Par., County Kent, Eng.; bride was a dau. of Margaret UNDERWOOD and the rel. of (1) Dr. James TAYLOR, (2) Capt. Francis SLAUGHTER; she mar. (4) the Rev. Amory BUTLER; acc. to Eaton, the bride was the sis. of Edw. ROWZEE; (RapC rB 1656-64:260; Avant:64 et seq.; Eaton:35; Underwood:91, 94; VG 38:1:22-23)
p. 304	ROWZIE, Edward (ROWZEE, ROWZEE) & PIERCE, Mary; bef. 20 Feb 1701; groom was on the 1704 ECRR; bride was a dau. of Col. Wm. & Sarah (UNDERWOOD) PIERCE of Nomini, CP (d. WC 1702); she mar. (2) Dr. Hen. OSWALD (d. 1726) of EC; (EC WB 4:146, 276a; WC OB 1698-1705:129; DW 3:2; 50, 243; MRC:174; ECRR:403; Underwood:94)
p. 321	SLAUGHTER, Capt. Francis & TAYLOR, Elizabeth (wid.); well bef. 5 Jan 1663/64; bride was a dau. of Margaret UNDERWOOD (acc. to Slaughter:307, she was a sis. of Col. Wm. UNDERWOOD); she was the wid. of Dr. James TAYLOR; she mar. (3) Col. Jn. CATLETT and (4) the Rev. Amory BUTLER; (RapC DW 1665-77:136; Avant 3:620; Slaughter: 307; Sweeny:11; Underwood:94)
p. 345	TAYLOR, Dr. James & UNDERWOOD, Elizabeth; well bef. 5 Jan 1663/64; bride was a dau. of Margaret UNDERWOOD; she mar. (2) Capt. Francis SLAUGHTER, (3) Col. Jn. CATLETT, and (4) the Rev. Amory BUTLER; (Underwood:94)

2.  George Norbury MacKenzie.  1907-12.  Colonial Families of the United States of America.  7 vols.  Vol. I by Grafton Press, New York; Vols. II-VII by Seaforth Press, Baltimore, MD (republ. 1966/95 by Genealogical Publ. Co., Baltimore, MD; Broderbund CD-189; online at Ancestry.com).

v. 5 p. 460

CAPT. FRANCIS SLAUGHTER, b. 1630; d. 1656; Captain of Militia, Justice for Rappahannock, Virginia; m. 1652, Elizabeth UNDERWOOD, dau. of Col. Wm. UNDERWOOD and Margaret, his wife. ISSUE   I.   FRANCIS, b. 1653, only child.

3.  Anon.  1902.  "Underwood."  William and Mary College Quarterly Historical Magazine.  1st series, vol. X, pp. 275-276 (reprinted in 1982 as pp. 247-248 in Vol. 5 of Genealogies of Virginia Families by Genealogical Publ. Co., Baltimore, MD; online at Ancestry.com):

v. 5 p. 247

UNDERWOOD.—Capt. John Upton was commander-in-chief of Isle of Wight county, was a member of he House of Burgesses, and muster-master-general.  His will, dated 16 Jan., 1651-'52, was proved in Isle of Wight county December 16, 1652.  (Vol. VII., p. 220.)  His son, John Upton, died without issue.  His widow, Margaret Upton, had issue by a former husband (1) William, (2), Elizabeth, (3) Sarah, (4) Margaret and (5) Ann Underwood.  She removed to Lancaster county, and married, thirdly, in 1657, Thomas Lucas, Sr.  The will of Thomas Lucas was dated Dec. 14, 1669, and was proved March 14, 1673.  (Virginia Magazine, III., p. 61.)  Of the daughters, Elizabeth married, first, Capt. Francis Slaughter, of Rappahannock county, who was a justice, and died about 1657.  Issue, Francis Slaughter, Jr.  She married, secondly, Col. John Catlett, of Rappahannock county.  She married, thirdly, in 1672, Rev. Amory Butler.  Her will is on record in Tappahannock, dated May 17, 1673, proved June 16, 1673.  (Virginia Magazine, III., 64.)...

4.  Mary Denham Ackerly & Lula Eastman Jeter Parker.  c1930.  Our Kin:  the Genealogies of Some of the Early Families Who Made Histor in the Founding and Development of Bedford County, Virginia.  J.P. Bell, Lynchburg, VA (online at Ancestry.com):  this is the most fact-filled genealogy -- need to extract.

5.  WorldConnect / Ancestry World Trees (online at RootsWeb.com/Ancestry.com).