The correspondence, said to have been written by Jane Hagan last year, just weeks before she and her partner Terence Horan were sacked by Whyte, stated that he had "been behaving rather badly" and was making his living "mostly by dubious means".
It also contained a claim that Whyte had bankrupted his father three times.
Ms Hagan and Mr Horan deny stealing thousands of pounds worth of goods from Castle Grant in Grantown-on-Spey.
They are on trial at Inverness Sheriff Court, where Whyte, 42, was asked yesterday by fiscal depute Heather Swan to read out the three-page letter as it contained what appeared to be a confession by Ms Hagan that they were taking items from the castle because they had not been paid and feared for their future.
The letter, which was allegedly written by Ms Hagan to a friend last June, says: "Our boss has been behaving rather badly. We have always known he is not the nicest person in business and his living is mostly made by dubious means.
"Talking to his wife not only confirms our suspicions that he is not a good man but we find he has treated her and her family in the most appalling way.
"He has bankrupted his own father and due to this he has lost his home, not once but three times, and still he uses him to do his dirty work."
The trial was in its second day yesterday after being interrupted last month due to a lack of court time.
Sheriff Jamie Gilchrist heard that a container had been rented by 54-year-old Mr Horan and had been filled with a huge array of household items, some belonging to Whyte and his estranged wife, Kim.
He and his wife had separated three years earlier and last year he noticed some items were missing from the castle, including a large kitchen table.
When Whyte returned from a holiday, he discovered more things missing and, on checking with his ex-wife, found she had not taken them.
He made a list and alerted police. His housekeeper and caretaker were nowhere to be found and he had to call out a locksmith to get inside.
He and his then girlfriend searched their quarters, and he found the letter, allegedly written by Ms Hagan, apparently confessing to the theft. A search warrant was obtained by police, and many of the items were found in a storage container in Grantown- on-Spey, amongst other items belonging to Ms Hagan and Mr Horan. Whyte told the court: "I did not order their removal, nor did I instruct anyone to store them or hide them."
Whyte also denied tampering with the letter. "I didn't doctor it, edit or make changes to it."
The trial was adjourned until October 23, as there was not sufficient time for defence lawyers to cross-examine Whyte.
Whyte then moved from the witness box to the dock to answer a contempt of court charge for failing to appear for the trial's opening on June 10, prompting a warrant for his arrest that was not executed.
Whyte's lawyer Paul Kavanagh apologised on his client's behalf and said the failure to show was a genuine misunderstanding.
Sheriff Gilchrist decided to take no further action, but warned Whyte that he had to attend court to finish his evidence.