3 links
Bartleby.com: Kyd References
bartleby.com
T.S. Eliot's essay on Hamlet from "The Sacred Wood" touches upon Kyd and "The Spanish Tragedy."
Legends: Shakespeare's Stories: Hamlet
legends.dm.net
Briefly discusses The Spanish Tragedy as a source for Hamlet.
Renaissance Tragedy
english-literature.org
Comparative essay of Shakespeare's Hamlet and Thomas Kyd's The Spanish Tragedy.