I’m writing a non-fiction book. The book is based around a blog I wrote. There were comments on the blog from family and friends, do I need to get their permission to use these comments or can I just put them in the book? Many thanks, Kate
If the blog does not have Terms of Service which require commenters to surrender their rights to the website owner, it doesn't matter who owns the website; it's still the commenter's intellectual property. They are choosing to have it distributed via the medium of your website, and they can revoke that permission.
Without a ToS clause, you do not have a license to distribute their IP via a different medium. If you do not or cannot get permission, you have to resort to the doctrine of Fair use.
An attributed quote will be either fair or infringing. If it's short and part of a discussion, it's probably fair, especially if you paraphrase. If it's a short story they posted in the comments, it's probably infringing. Microfiction is complicated.
Under no circumstances can you present their content as your own, or use it for purposes other than "fair" (commentary, criticism, parody). For example, you can't use comment in your novel as either narration or direct speech.
Kate, you can assume that a certain amount of reproduction from your web site to your book is fair use. They did, after all, post them in a comment on a public place owned by you. If their writing is substantial (i.e., pages of material), however, you may want to seek permission.
Another way around this is to paraphrase their comments rather than quoting/copying their writing exactly. Depending on the focus of your book, this might work for you better. A judicious use of original quotes and paraphrasing can give people a feeling for their writing style (if that's needed), but stay within the fair use provision.