Found an amazing source for a presentation. How do I avoid plagiarising it?
Acknowledge the influence clearly and appropriately.  Precisely what this means will depend on the specifics:


If you use direct quotes, then show them clearly as such — i.e. in quotation marks, or some visual environment clearly marking them as quotations.
If you’re using figures, acknowledge this in captions: “Taken from Miaow 2011, p.73”, or “Based on Miaow 2011, Fig 12.2”, or similar.
If you’re not directly quoting text or re-using figures, but major aspects of your overall organisation have been influenced by the book, then add a note saying this at the beginning or end: “Presentation heavily influenced by Miaow 2011, Ch.12” (or “based in part on”, “greatly indebted to”, etc.)  The bigger the influence, the more prominent you should make your note.
If your work will be advertised via an abstract, or similar brief description, and if the influence is particularly large — e.g. forms the main body of your presentation, not just one of several sections — then it may be appropriate to mention it in the abstract: “(This is an expository seminar, based in large part on Miaow 2011.)”


As long as the acknowledgement is prominent and accurate, then you are not plagiarising.  However, a couple of related issues to be careful about:


In some contexts, there’s an expectation of originality/novelty in your work.  If too much of your work is heavily based on earlier work of others, then it may not be considered suitably original or novel for these venues.
Copyright may also be an issue.  I’m not legally very knowledgable, so take this with a large grain of salt; but roughly, I think it’s likely to be a concern if you are either directly quoting a significant proportion of the text, or directly reusing figures/images.