Look at the top spine edge of most hardcover books and you'll see the signatures. Perfect binding is a cheap form of binding used in magazines, paperbacks, and many book club hardcovers. The pages are lain in a pile and glue is applied to the spine - like a note pad. With any significant use, the book splits open at the spine. Shaken and moused boards also refer to condition.
Boards are the covers on a hard bound book.
Moused is a vivid term indicating that the cute little critters have had a picnic - with the book forming the main course. They love the glue in the bindings. Shaken means that the pages have become partially separated from the boards. Shaken boards are one step up from detached boards. Normally, writing is a serious defect.