Avoid photographs that were issued after a photographer died. Most such portfolios are not considered to be collectible by the vast majority of today's serious collectors and have little if any secondary market (read auction resale) value. The reasons are several:
1.) Since there was no review of the prints by the photographer, there are more questions as to whether or not the print meets the intentions of the artist.
2.) Most of these later printed portfolios were not very limited in quantity--some being as high as 1,000 copies made. Rarity is always a factor. And most estates are frankly greedy.
3.) Unsigned late prints are always susceptible to fraud and greed. There is no guarantee that the estate won't print more.
4.) The best guarantee on a print maintaining its value is that it is vintage, signed and rare.
5.) Print quality on these type prints are sometime poor imitations of the originals, as are even late signed prints in many instances.
6.) Track record: frankly, with the exception of Diane Arbus prints made after her death, most such prints have flopped badly, rarely increasing in value. And even in the case of the Arbus prints, her own vintage work has gone up more than the later prints issued by her daughter Doon Arbus, although the prints by Neil Selkirk are decent and the estate put a genuine limitation on the prints. But don't expect the exception to BE the rule.