Don Henley and Glenn Frey followed a routine while writing some of the most emblematic and enduring songs of the 1970s.

The men, who co-founded the Eagles, would rent a house and bring in a piano and guitars. The two would rise in late morning — “musician time,” Mr. Henley testified in a Manhattan courtroom on Monday. They would make coffee, then have “philosophical” conversations and begin trying out riffs and discussing “song titles, subject matters, concepts,” he said.

Mr. Henley paid particular attention to lyrics, crafting and refining them on legal pads. The pages came to have a deeply personal meaning, and Mr. Henley said he saved them inside a barn on his organic farm in Malibu, Calif.

Now they are at the center of an unusual prosecution in New York State Supreme Court. A rare-book dealer, Glenn Horowitz, is accused along with two other men of conspiring to possess stolen property — some 100 pages of Mr. Henley’s handwritten notes and drafts for hits like “New Kid in Town,” “Hotel California” and “Life in the Fast Lane.”

Prosecutors say the notes were stolen decades ago by an author who had signed a contract in the late 1970s to write a book about the Eagles. The author, Ed Sanders, has not been charged. He sold the documents in 2005 to Mr. Horowitz, who in turn sold them to the two other defendants, according to the Manhattan district attorney’s office, which began investigating after complaints by Mr. Henley.

Testifying during a break from what the Eagles have billed as their final tour, Mr. Henley said that he was alarmed in 2012 when he first learned that a few pages of his “Hotel California” notes had been put up for auction online.