I'm using the beta version of a book that was given to us as the course material by the author (the instructor of the course), for a presentation. How should I reference it? There are older versions of it already published, does it make sense to cite those and mention that I used the beta version of the soon-to-be-published edition?
The most important thing about referencing any source of information is that your audience can find the source, either now or (if the source hasn't been published yet) in the future. Lots of referencing questions become easier if you stick to this principle.
If there are specific standards in your field – for example if you are doing this presentation as part of the assessment for a taught course – then you should stick to those standards. Many referencing guides have sections on referencing 'forthcoming' or 'in press' material. For example, the US National Library of Medicine has detailed instructions for this. Similarly, the APA Style Blog has advice on citing material that is still being prepared, that has been submitted but not accepted for publication, or that has been accepted but not yet published.
If a search for 'forthcoming material' on the website of your preferred or mandated referencing style doesn't produce any useful results and the presentation is for credit, your university library will probably be able to provide advice. If they can't help you, simply asking your instructor will also probably produce a useful answer.
If there are no particular standards for you to follow (e.g. it's a conference presentation and the organisers don't mandate a particular style), you can stick to the principle of making sure your audience can find a source by simply choosing a referencing style and sticking to it. For example, one of the examples on the NLM instructions page is:
Posner MI, Rothbart MK. Educating the human brain. Washington: American Psychological Association. Forthcoming 2007.
As long as your audience can find the book, that's what matters most.