Eater by one of my favourite science fiction writers from the 1990s, Gregory Benford. By way of summary: astronomers are surprised to detect what turns out to be a small black hole entering the solar system, but even more surprised when it communicates a desire for conversation with the Earth. But dreams of a scientific bonanza from this ancient galactic traveller quickly turn sour as its requests for information turn into demands for much more.
What to say? Well, a great premise ruined by clumsy execution and an excruciatingly bad ménage à trois. Though borrowing liberally from Fred Hoyle's classic novel The Black Cloud (a previous read), it heads off in its own interesting directions before becoming bogged down in some painfully bad "relationship" nonsense between the central astronomer characters. As the novel progresses it also relegates the Eater, which was shaping up to be the most interesting character by a country mile, into a cartoon villain whose hinted subtleties get completely lost in a ridiculous plot where humanity, well on its way to getting humbled by the cosmos, inexplicably and implausibly turns the tables. The only interesting bits are those where the Eater philosophises away, but this is but a faint echo of Benford's alien intelligences from his Galactic Center Saga. There, he did a creditable job of making believable the thoughts of beings on a scale epically removed from humans. Here, his efforts to do the same are simply wasted when interspersed by the thoughts of beings that seem cribbed from a bad "young adult" novel.
Disappointing from someone such as Benford.