Records relating to the Barony of Kendale, CWAAS, William Farrer & John F. Curwen (editors) are available on British History Online (also Supplementary Records)
Cumbria County History Trust has published a "Jubilee Digest" for the township of Crosthwaite with Lyth.
Magna Britannica et Hibernia.Volume 6: Westmorland by Thomas Cox (Vicar of Bromfield, Essex) 45 pages, printed in 1731. Transcription by Sarah Reveley, Joan Fisher and Lisl Schoenwald. (Rootsweb Westmorland Listmembers) (c) 2003
"Crossethwaite, an Hamlet, whose Manor was, 30 Hen. III. The Possession of William de Lancaster, Baron of Kendal, who died then possessed of it; but leaving Agnes de Brus his Wife surviving, she had for her Dowry an Assignation of this Manor and divers others which she held for her Life. Tho whom it passed from this Family, we do not certainly know, but find Robert de Thweng dying possessed of it, and other Estates in this County 48 Edw. III. He had no Issue, and so his Estates descended to the Heirs of his three Sisters, Lucy, Margaret, and Katharine; but to which of them this Manor fell in the Partition doth not appar; for we observe nothing of it, till John de Beaufort, Marquis of Dorset, died possessed of it 22 Hen. VI. And leaving it to his only Daughter Margaret, with his other great Estates, she by Marriage carried them to Edmund of Hadham, Earl of Richmond, whose Son Henry by her became King of England, by the Name of Henry VII. Lych, the Lordship of William de Lancaster, Baron of Kendal, the Third of that Name, who dying 30 Hen. III left Agnes his Wife surviving, who had this Manor and divers other Estates assigned for her Dowry; the Rest passed to his Heirs, who were his Sister's Children, of whom Peter de Brus, the Son of Helewyse, his Sister, had the Barony of Kendal, to which this Manor belonged. From his Family it passed by the female Heir also to Robert de Ros, who was on that Account stiled Baron of Kendal. Le Lythe, the Lordship of Thomas de Theweng, who died seized of it, and divers other Estates, 48 Edw. III leaving them to the Descendants of his three Sisters, viz. Lucy, the Wife of Sir Robert Lumley; Margaret, to Sir Robert de Hilton, and Katharine of Sir Ralph D'aubeny; but to which of them, upon the Partition, this Manor fell, we cannot discover. It is an Hamlet to Helsington."