When it comes to the world of Rare Books, there is a lot of jargon that we don't use in our everyday language. Books, for example, specifically mean something that is printed, in the rare books world. Printing was introduced to the Western world in 1454 by Johannes Gutenberg, however, printing existed in East Asia roughly 400 years before that.
So whether it be on a printing press in the 15th century or published by Penguin Books in 1987, if the item you're looking for has been printed, the term you'll look for and use is 'book.' Another example of a "book" is a broadside. The definition for broadside is in the terminology section to the right. Even those these are only one sheet of paper, they are part of library collections, not archival collections because they are printed.
One way to know if something is on the library side or the archives side is it's catalog number. Books will be individually catalogued and have a specific number per volume. Whereas all items in an archival collection--including any books--will share the same collection number.
Manuscripts refer to something that is handwritten. The word comes from Latin: "manu" for hand and "script" for write. Manuscripts can come in a variety of formats. They can be bound books, or single leaves; they can even be petrified clay.
The most prized type of manuscript illuminated manuscripts on parchment. These, sometimes referred to as medieval manuscripts, can be from anywhere between 500 and 1400 CE. Sometimes these are bound books of hours, or single leaves that have been ripped from their previous bindings. Sometimes medieval manuscripts will have been recovered from the binding of another book. While they may not have always been highly valued, these are heavily sought after today.
Manuscripts can also be, however, literary drafts. When there is a significant author, like Kurt Vonnegut for example, who donates their works to an institution, this might include drafts that were printed on a typewriter or, if they're recent enough typed on a computer and printed, and these are still considered manuscripts. Part of this is because they are part of what is already a manuscript collection, and the other part that distinguishes them from printed books is that these drafts were not intended for publication.
Demonstrated by the explanations to the left and the glossary below, there is a lot of room for overlap between "books" and manuscripts. You will find that bound manuscripts are heavily referred to as "books" (because they were the only books before printing was invented!) and printed books referred to as codices (plural of codex). Sometimes book dealers will get even more specific and refer to a handwritten material as a "holograph manuscript" which is basically saying "a handwritten handwritten document."
When it comes to collections, there are also plenty of "manuscript" collections that include printed books in them. This is usually the case that the collection is from a specific person and includes some of the books that they owned. For provenance purposes, these stay with the original collection rather than being added into a library's overall book holdings.
All of this to say, libraries and archives and books and manuscripts can get very confusing, but that's why there are people staffing them. Hopefully this LibGuide helps clear some things up, but if it doesn't, never hesitate to ask questions. Librarians are huge nerds who love talking about their collections, you'll never be a bother for being curious.
Here is some useful terminology when looking at descriptions of materials in the world of rare books and archives: