Many books will have the same classification number and will arrange by author within the number. These should shelve separately, preferably on counter height shelves, nearer a child's eye level. A reference book is identified as one used for reference and not meant to be read through, like dictionaries and encyclopedias. No book cards are used for these, as they remain in the library. Before assigning a class number, skim the book to be sure of its content.
Titles are often misleading.
Tables of contents and chapter headings are good clues. However, instead of author letters, letters for the subject are used. This keeps biographies of the same person together on the shelf. On the title page and on the spine of the book. On the book pocket and the book card.