Hi Uli,
I don't know if a new version will be available next months.
In books about ElasticSearch, there are important parts that change for
every ElasticSearch version and some in depth parts related to Lucene
(analyzers, tokenizers, way to manage data) and architecture that don't
change.
I was asked to write my third edition of the book targeting 5.x.
The 5.x version have a lot of big changes (ingestion, "filter removing",
internl refactory, no more rivers, ...) so if you need a book in short
range take what do you like, otherwise it's better to wait books for 5.x
release that will be available in 5-6 months.
A tip for everyone: packetpub offers everyday a free ebook at
https://www.packtpub.com/packt/offers/free-learning some of ES 1.x books
are already been given free (such as mine).
Regards,
Alberto