Writing in the margins of books, though it may be a bibliophiles nightmare, is a time-honored way to to keep notes on your reading. But they are not easily searchable. Adding notes and annotations to ebooks is easy and they are certainly searchable, but not all of your books are in electronic form. Certainly you can take notes about any book you are reading using a text editor or word processor on your laptop. You could even take notes on your iPad or iPhone using any note-taking app.
Enter Margins, a book notes app. In true app form, it takes a task that could be handled in other ways and builds it into a simple program that makes it portable as well as organized. You add your book by manually entering author, title, publisher, etc. or by entering the ISBN. Once your book is added you simply add your notes. You can input the page number, a quote from the page, and your own notes. These all become searchable fields. Simple.
Now for the downsides: the app is showing its age; it is for iPhone, so you can run it on an iPad but in small form (though the 2x view is crisp and frankly easier to type in). You can send your notes out by email but there is no cloud-based saving. It is built by Architechies, better known for their converter apps, but has not been updated since 2011. Nevertheless, it does what it says it does,