I haven’t yet read the book. But I listened to a recent interview of Breyer on NPR’s Fresh Air, read this review  of the book by Cass Sunstein, and read the New Yorker essay by Jeffrey Toobin. The last two emphasize Benjamin Constant’s  influence upon Breyer’s thinking – in particular, Constant’s description of the “liberty of the ancients” as “an active and constant participation in collective power."
Breyer sees the Constitution as a device aimed largely at increasing democratic participation – “active liberty,” as he calls it. This vision, in turn, leads Breyer as a member of the Supreme Court to defer greatly to legislative bodies even when they violate individual property rights and freedom of contract.
But I believe that Breyer and Sunstein each seriously misreads Constant. In the fabulous and readable and important 1816 essay  in which the above quotation appears, Constant contrasts the “liberty of ancients” with the liberty of we moderns. My reading of Constant is that modern liberty is not only significantly different from ancient liberty, but that ancient liberty is inconsistent with modern liberty.
Here, for example, is the fuller context from which the above quotation is extracted:
It follows from what I have just indicated that we can no longer enjoy the liberty of the ancients, which consisted in an active and constant participation in collective power. Our freedom must consist of peaceful enjoyment and private independence. [emphasis added]
And here again is Constant, later in the same essay:
Individual independence is the first need of the moderns: consequently one must never require from them any sacrifices to establish political liberty. It follows that none of the numerous and too highly praised institutions which in the ancient republics hindered individual liberty [such as requiring participation in public debate] is any longer admissible in the modern times.
I concede that Constant left himself open (but just barely) to a different interpretation. He says at the end of his essay that we moderns “must learn to combine” ancient and modern liberty. But this claim – quoted favorably by Cass Sunstein (and, I gather, by Breyer) as endorsing a democracy-enabling interpretation of the Constitution – is on its face inconsistent with the rest of Constant’s essay. What Constant means – what he says clearly and often elsewhere in the same essay – is that the freedom of ordinary people to participate in political deliberation and choices is a means of protecting modern liberty. Unlike for ancient peoples, political participation is no end in itself, no significant source of gratification for we moderns as it was for ancient peoples.
The liberty that matters for us, the ultimate liberty, is individual liberty. It is the liberty that Constant rightly understood to be modern.