To answer your specific question, the current downloadable JIRA version is 7.2.4 (as of today, 25th November 2016). The Cloud version of JIRA may be a minor increment ahead (e.g. 7.2.5), but in general terms it will be, as Nic mentioned, very similar to whatever the current downloadable version is.
So if you read a book on JIRA 7 you'll be reading about the latest major version, and the core information will match the latest version of JIRA Cloud. You can see the Cloud release notes here to find out what's changed between 7 and 7.2.4
It doesn't matter, it's always "latest".
You'll generally find that Cloud and Server are similar enough in terms of core functionality, that you can work out an approximate version from looking at the latest available server version you get if you download from https://www.atlassian.com/software/jira/download
Cloud and Server editions are likely to diverge somewhat in the future though, and it's important that you understand that there are two huge differences. On Cloud, you don't get "System administrator" rights, so you can ignore any sections of books that mention that, and then there's the add-ons. Cloud only supports a (shrinking) handful of server add-ons, plus Connect add-ons. Server does not support Connect add-ons.
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