Hi Job, 'All business' means 'serious', or 'only concerned with whatever business is to be transacted'. We often use the phrase about someone who is unwilling to joke or chat during a business discussion. In your quotation, it seems to mean that Reagan was just normally occupied with whatever business he had with Ruth Marone; he was not strange about it.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.