If you're looking to read 'Things Not Seen' online, I've got a few reliable routes I use that usually work out. Public libraries are the easiest — search your local library's catalog and you'll often find an ebook or audiobook copy available through Libby/OverDrive or Hoopla. All you need is a library card number and PIN, and you can borrow it for a set lending period; I love this because it keeps things legal and free, and the apps sync across devices.
If a library copy isn't available, you can buy or rent it: Kindle, Google Play Books, Apple Books, and Kobo sell ebooks, and Audible or Libro.fm usually have audiobook editions. Scribd sometimes carries it if you have a subscription, and Google Books will often show a preview so you can sample chapters before buying. Avoid dubious sites offering free downloads — this is a copyrighted book, so piracy is both risky and unfair to the author. Personally, I usually check my library first and then buy the version that fits my reading habits; it's nice to support the book if I know I'll revisit it.