Which is the best edition of An Experiment with Time to get? It's a tricky question. Dunne deliberately kept bringing out new editions, to keep people buying them. He also became obsessive about keeping up with criticisms and brought out many more updates, which his publisher disguised as reprints so that the bookshops would not return unsold copies of the previous one.
The end result is an eyewatering choice. I currently have sixteen assorted editions and impressions/reprints on my shelf and my collection is far from complete. All the complete books have much the same story to tell, unless you have a particular angle of interest. If that last is the case then the best for you will depend on what you want it for. Here are some suggestions:
First Edition, A & C Black, 1927 (MCMXXVII). This is really rather primitively laid out by the publisher. It has neither Table of Contents nor Index. Collectors will want pristine copies - at a price! Historians will be more interested in the cheaper but scruffier end.
Third Edition, Faber & Faber, 1934 (MCMXXXIV). For literary studies, this is the definitive classic. It was the first to become a top seller, and in its several impressions was the one owned by JRR Tolkien, CS Lewis and other influential writers. See also the Papermac edition.
New (Sixth) Edition, Faber & Faber, reprinted 1948 (MCMXLVIII) (or later). Note this is the second "reprint" of the "new edition". It is the last version containing any new changes. All subsequent Faber reprints were of this version, and will do just as well. These and the intermediate editions tend to be a bit cheaper than the Third.
French edition, 1948. Le Temps et le Rêve is translated by Eugene de Veaux and published by Seuil. It is the closest we have to Dunne's final intentions. The two friends worked together closely, with the bilingual de Veauce putting forward various improvements that Dunne wanted to adopt in his own language. de Veauce was able to incorporate more of them into his translation that the increasingly infirm Dunne was in the original English, which was "reprinted" that same year.
Fate, November and December 1952. Condensed extracts from Dunne's accounts of his dreams. Illustrated with original artwork. A bit of fun, though the gratuitous cover art offends today's gender equality. Useful if you are studying his influence on postwar F&SF writers.
Papermac, 1981. Fully reset printing of the classic Third Edition, this one also has an introduction by Brian Inglis, which marks it out as my top recommendation.
Hampton Roads, 2001. Fully reset printing of the last edition, this one has a preface by Russell Targ, who is rather more focused on parapsychology as a field of study than is Inglis.
If you prefer an eBook, I'd recommend the Second Edition available from the Internet Archive as a free download, in various formats.
There are also of course several unauthorised editions around today, published as print-on-demand or even eBooks. These are all reproductions or automated resettings of older editions, and are of very variable quality. But they are readily available and can be affordable. On the other hand, downloads cannot always be trusted to be free of malware. If you put your cash into one of these, don't blame me for what you get.
If your language preference is for French (as above), Italian, Swedish, Danish or Spanish, then you are in luck; look up whichever means more to you out of Le Temps et le Rêve, Esperimento Col Tempo (two editions, the second with Inglis also translated), Experiment Med Tiden, Et Forsøg Med Tiden or Un Experimento con el Tiempo (with prologue by Jorge Luis Borges).
Updated 1 Sept 2021