Edition: HarperCollins, 1987
Review number: 166
In terms of publication date, Intervention falls between the Saga of the Exiles series and the Galactic Milieu trilogy; in terms of the internal chronology, it comes before either (time travel making later events in the lives of May's characters happen millions of years before earlier ones). It is May's longest work, and has an expository character, filling in much of the background of the other series.
Intervention is the memoirs of Rogatien Remeillard, one of the first of that family to discover their mental powers, and a fond great-uncle reluctantly pushed into action to manipulate his family toward greater mental operancy. (The pushing is done by an invisible presence, which Rogi calls the Family Ghost.) As the mental powers of human beings over a century or so, the excitement of watching aliens mounts; they are waiting for a certain level of operancy to be reached before making their presence known and inviting the human race to apply for membership of the Galactic Milieu.
Intervention is long, sufficiently long that I think the US edition was split into two parts published separately. May manages to keep the interest of the reader, but the book's character is definitely explanatory and is much more aimed at those interested in the two related series rather than those yet to read them.