Topic and title
There are almost no limits when creating books from Physicswik icontent. A good book focuses on a certain topic and covers it as well as possible. A meaningful title helps other users to have the correct expectation regarding the content of a book.
Books should have a reasonable number of articles. One article is not enough, but books that result in PDFs with more than 500 pages are probably too big, and may even cause problems on older computers.
Saved books can be modified to include an introduction. The introduction text can be stored on a subpage of the book. See Help for experts on how to include additional pages manually.
If your book has a clear focus, meaningful title and is a reasonable size, consider adding it to the list of community books. You can either save it directly to Book:Title, or move it there from your userspace.