Am I allowed to use draft chapters of an early version of a book this way? I am writing a book and am seeking feedback on different chapters before submitting for final publication.  Can I use the draft chapters as conference papers or at visiting lecture seminars?
That is up to the book publisher. Some will be fine with it, others not. Copyright might be one (blocking) issue. If you have given up copyright to some conference, then some publishers will reject your proposal, since they want to hold it and it is no longer yours to give. Lectures and such are no real issue, since you retain copyright.
Another thing that might influence a publisher (around the copyright issue) is how different the book chapters are from the earlier material. But even then, you have to be careful about self-plagiarism since &quot;repeating yourself&quot; in one paper of something in another needs to be handled delicately and with full disclosure.
Just make sure you retain rights (possibly a license) to anything that you publish and want to include in a book later.
You can, of course, publish many things on a topic in conference papers that don't conflict with any rights that you might need in order to publish a book.