If you have over 100 books, you've probably tried to organize them in some way, either by color or topic or size. Why not organize them like libraries do? Learn to organize your books by the Dewey Decimal System, used in libraries around the country.
Sort your books between fiction and nonfiction.
Look on the inside of all your non-fiction books. Most will have a copyright page near the front which has copyright information, publishing information, and a section with cataloging information. A few lines down in this area will be a set of numbers that represents the Dewey Decimal classification for the book. One book, for example, may have the number 303.4. Use that number, along with the first letter -- or two or three -- of the author's last name, on your label, like this: 303.4 D or 303.4 Den. This helps if you have many books on the same topic.
On your label, write or type the book's number. Put the label on the spine of the book about a half-inch from the bottom. Align the label so the type on the label matches the type on the spine of the book. If the spine is too skinny, you can put it on the front cover in the bottom left corner.
Cover the label with packing tape so it won't peel off easily. Do this for each of your books that have Dewey Decimal information.
If you have books that don't have cataloging information in the front, you have to get a little creative. If a book is very similar to another -- two Thai cookbooks, or two books on childbirth, for example -- you can use the same number for both, changing the author initials. You could use an online resource to find the topic of your book in the Dewey Decimal Classification System. You also could log on to your public library catalog, see if it has your book, and verify its Dewey Decimal number there.
For fiction, sort books by the author's last name. Type either the full last name or the first few letters of it on the label, and stick the label on the book.
Once you've labeled all your books, sort them onto your shelves. Use one shelf or section of a shelf for the 100s, one for the 200s, etc. Once you have a handle on how many books you have, you can shift them as needed. Soon you'll have all your shelves organized, and your books will each have a place.