Poul Anderson Paperbacks

A neighbour who is a former sf bookseller has just brought me a bagful of Poul Anderson paperbacks, all in good condition, none that I do not already have but some can replace my copies in poor condition. I already had Planet Of No Return (London, 1971). It begins with a quote from Ecclesiastes, which I have probably noted before. 

The bag of books included Question and Answer (New York, 1978). The author's Introduction explains:

a professional scientist designed the planet Troas;

Isaac Asimov, James Blish and Poul Anderson were each asked to create a narrative about Troas;

originally intended to be published in a single volume, the three works were instead published separately over time;

Anderson's was serialized in Astounding, then published as a paperback entitled Planet Of No Return;

Anderson has now restored the original title. 

Does anyone know which are the Asimov and Blish works about Troas?

(The original shorter version of Blish's A Case of Conscience was produced in the same way. Apparently, there is at least one other story about the planet Lithia by a different author out there somewhere.)