AS SUCCESSOR to the late Van de Valk, that estimable policeman prematurely and unforgivably killed off by his creator a few years back, Henri Castang has had a hard act to follow.

Author Nicolas Freeling has said that he had to switch heroes to free himself as a writer. In The Night Lords (Pantheon, $7.95), he dispatches Castang, a police inspector in a large French provincial city, on his fourth assignment. The corpse of a naked young woman has been found in the trunk of a Rolls Royce on the parking lot of a three-star restaurant. The investigation becomes entangled in diplomatic niceties when the owner turns out to be a titled English jurist touring France with his family. Then a tramp and the harmless old caretaker at Castang's apartment houses are found dead, apparently suicides.

As Castang works on these separate cases, he indulges in his "mania for understanding" just why people do what they do. Freeling writes complex mysteries that have to be unpeeled layer by layer. The effort can be richly rewarding, at other times, equally frustrating. Castang frequently suffers attacks of acute introspection that are likely to try the readers' patience.

However, with each new book, Castang is growing in character. So are his wife, Vera, the former Czech athlete still recovering from a crippling accident, and the supporting cast of Castang's colleagues. Richard, Castang's boss, has a prominent role in The Night Lords and comes up with this succinct assessment of Castang: "the brightest of them, as well as sensitive and imaginative enough for two, but unreliable because of soft spots and apt to go dreamy at the wrong moment."