Topic & Title
There are almost no limits when creating books from Wikipedia content.
Good books are focused on a certain topic and cover 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 and books that result in PDFs with more than 500 pages are probably too big, may even cause problems when opened on older computers.
Stored 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.
Size of the PDFs
On 'stored books-pages a template stored book is automatically included. This template can be assigned the number of pages in the PDF, which will then be displayed at the head of the book-page.
If you created a nice book, consider to add it to the list of community books.
You can either store it (from Special:Book) directly as a subpage of Wikipedia:Books or move it there if it was created in your user namespace.