Specification for Java 7 is available on the Oracle site, but I can't find Java 8 specification anywhere, although downloads of beta versions of Java 8 are available on the internet.
Do you have an idea: is it already written and where can it be downloaded? If the specification isn't available, maybe another document which can act as (or come close to) the specification?
Now that Java 8 is officially released, the Java Language and JVM specifications can be found on Oracle's website.
Update: thanks to @MadChuckle, link to final release.
According to the project's page, a public review will start in October 2013. In the meantime, you can follow the progress on the various mailing lists provided on that page or have a look at the early draft.
At this stage, the early draft mostly refers to individual sub-projects, so if you want to look at the changes brought by the lambdas for example, you would need to find that sub-project's documents which should indicate the impact on the JLS (not sure if that old draft is the latest - probably not).