The value of the volume is the uncollected half. The five books it reprints are all in print separately; what is not otherwise available is the thirty-seven years of magazine work around them, from the first pieces he published in his early twenties to essays finished the year it came out.
It is arranged chronologically rather than by book, so the individual collections are dissolved into the run. That is the argument the volume makes: read straight through, it shows a position being worked out and revised over four decades rather than five separate statements of it.
Baldwin was sixty-one and had two years to live. It is the last shape he gave his own work.