Caleb was a family friend of the Winchesters as well as of the Harvelles; according to Jo's blog, Caleb was a regular visitor at Harvelle's Roadhouse in the late 1990s. She called him "Uncle Caleb", which means they were probably well-acquainted with one another.
At some point, Jo wanted to start her own hunting journal, and Caleb tore out a page from his own journal and gave it to her, so she would know what it should look like.
In "Asylum", Sam called Caleb for information but told Sam he hadn't seen John for a while and promises to let Sam know if he's seen anything.
He was the one to find Jim Murphy, a close friend of him and their friend John Winchester, after he had been killed trying to protect John's whereabouts from Meg. Caleb was also the one to call John and inform him of Jim's death -- something that brought John to tears. Caleb also informed John of sulfur that he found at Jim's church.
Caleb was captured by Meg in attempt to coerce John into handing over the Colt. When she put Caleb on the phone, he tried to warn his friend not to give in, but Meg slit his throat.