His book didn't say. However, the book The Siege of Charleston 1861 - 1865 by the director of the Charleston Museum, E. Milby Burton, does give a reference for it: "Battle and Leaders" Volume 1, page 103. I suspect Burton is referring to "Battle and Leaders of the Civil War" Vol. 1. That's possibly the five volume set that is listed on Amazon as "by Robert Underwood and Clarence Clough Buel, eds. (Peter Cozzens, ed. of vol. 5) Johnson." I don't have those books. It might also refer to an older "Battle and Leaders of the Civil War" published in the 1800s by the Century Press, but I suspect the former.
At various times in the past I have run into copies of army or government communications reprinted in newspapers that did not make it into the Official Records. The Official Records is a great resource, but unfortunately it does not contain all government and army communications of the period.
From Buchanan's letter in the OR you linked to [my bold and underline]:
These were the last instructions transmitted to Major Anderson before his removal to Fort Sumter, with a single exception, in regard to a particular which does not in any degree affect the present question. Under these circumstances it is clear that Major Anderson acted upon his own responsibility, and without authority, unless, indeed, he had "tangible evidence of a design to proceed to a hostile act" on the part of the authorities of South Carolina, which as not yet been alleged.
So, was Anderson basing his move on anything more than rumors and "Ha, ha, we're going to get you suckers" type of taunts and his own assessment of 'what if I were attacked here?' I think the latter. From Klein again:
... every time his officers had urged him to move the command to Sumter, Anderson reminded them that he had been assigned to Moultrie and could not vacate it without orders. Now he realized that no orders would be forthcoming and that Moultrie would be even more helpless if state troops seized Sumter and turned its guns on him. With an energy and decisiveness that surprised perhaps even himself, Anderson perfected his plan. He had hoped to make his move on Christmas, while the city was preoccupied, but rain forced him to wait until the next day.
After Anderson moved to Sumter, Floyd sent him a telegram on the 27th saying in essence, what the hey, you didn't have orders to move. Anderson replied that that was correct.
Buchanan also said in his letter you linked to [my bold and underline]:
But I acted in the same manner as I would have done had I enter into a positive and formal agreement with parties capable of contracting, although such an agreement would have been on my part, from the nature of my official duties, impossible. The world knows that I have never sent any re-enforcements to the forts in Charleston Harbor and I have certainly never authorized any change to be made "in their relative military status."
Seems to me that is consistent with Klein's account of Buchanan's response to Anderson's move in my post 1287 above.
Now back to my mother's inventory.
No, I think it's "Battles and Leaders of the Civil War", the four volume set that's been out for years and was a compilation of articles written by the men who actually fought there. It's available online here.
So, was Anderson basing his move on anything more than rumors and "Ha, ha, we're going to get you suckers" type of taunts and his own assessment of 'what if I were attacked here?' I think the latter.
Anderson didn't think so. In his report to Washington sent on December 27th he makes it clear that it was not a decision he reached lightly, and was made only because he was convinced that South Carolina forces were going to attack him. Link
Buchanan also said in his letter you linked to.
That is true, Buchanan never ordered Anderson to move his men. But the Secretary of War granted Anderson that authority in instructions delivered to Anderson by Major Buell: "The smallness of your force will not permit you, perhaps, to occupy more than one of the three forts, but an attack on or an attempt to take possession of any of them will be regarded as an act of hostility, and you may then put your command in either of them which you deem more proper to increase your power of resistance." Link
Anderson did not take his decision lightly. He had received reports of South Carolina militia mustering and scaling ladders being constructed. James Pettigu, a local attorney and Unionist, flatly told Anderson that Moultrie would be attacked. Had Anderson waited until an actual attack he would have been helpless. He made the only decision he could under the circumstances, and one he was authorized to make.