Short answer: almost definitely NO.

First Class Flat (= "First Class Large Envelope") rates are inexpensive, but books do not typically qualify as "flats" (flats are letters and other flexible materials, and Large Envelopes must exceed the size of letters, which are classified as a maximum of 6-1/8 inches by 11-1/4 inches).

First Class Flat rate is also known as "First Class Large Envelope" rate.   Size requirements to use Large Envelope rate:

• Must be rectangular in shape.
• Must have one dimension that is greater than 6-1/8 inches high OR 11-1/4 inches long
  • HIGH = the short side of your book (if you stood it up on a table, with its spine on the table surface)
  • LONG = the long side of your book (this is usually the spine side)
• Must be no more than 12 inches high x 15 inches long x 3/4 inch thick.
• Must have 1 inch flexibility in each dimension.
• Must be uniformly thick.

This information can all be found on the USPS.com website.  Even some USPS employees are confused about this: you may get information from individual USPS employees that conflicts with the official information.  Basically:

• If your book is longer than 6-1/8 inches on its spine side, but this is NOT the short side of the book, then the dimensional criteria have not been met for First Class Large Envelope (= First Class FLATS) rate.

• If your book is very long, and thinner than 3/4 of an inch and also flexible in all dimensions, you can use large envelope rates, but this would be an exception: a large thin spiralbound book might fit the criteria, for example, but not a typical paperback, and no hardcover of any size would be considered flexible, of course.

To decide if First-Class Large Envelope (= First Class Flat) Rate will work for your book:

• Check the dimensions (length/heighth):

• Cut a sheet of printer paper to 6- 1/8 inches by 11-1/4 inches. 
• Place your book on it so that the long side of your book is parallel to the long side of the paper, centering the book on the paper
• If your book overhangs the paper edge on either the long or the short side, you have passed the length/heighth criteria, and you should then check the thickness of the book
• If your book does NOT overhang the paper, it does not meet the heighth/length criteria, and needs to be sent package rate (First Class Parcel or Media mail).
• Check the thickness:
  • If it is greater than 1/4 inch but less than 3/4 of an inch, then it meets the thickness criteria, and you should then check the weight
  • it it is thinner than 1/4 inch or thicker than 3/4 of an inch it does not meet the thickness criteria, and needs to be sent package rate (First Class Parcel or Media Mail).
• Check the weight
  • If it weighs more than 3.5 ounces but less than 13 ounces, it meets the weight criteria, and you should then check flexibility.
  • If it weighs less than 3.5 ounces or more than 13 ounces, it does not meet the weight criteria, and needs to be sent package rate (First Class Parcel or Media Mail).
• Check the flexibility
  • If it is able to flex greater than 1 inch in all directions (the spine/binding usually prevents this in a softcover book--try to flex the two short sides to meet each other, in other words), it meets the flexibility criteria, and you can send this book using First Class Large Envelope (aka First Class Flat) rate.
  • If not, it does not meet the flexibility criteria, and needs to be sent package rate (First Class Parcel or Media Mail).

You cannot simply put your small book into a large envelope to try to qualify for Large Envelope rate--this will not work, since the package will not be uniformly thick.

• Also you run the risk of the loose envelope flap getting caught in postal sorting machinery and damage to or loss of the book inside.  We recommend that all packaging be taped down tightly around the book to prevent this.

In summary: The great majority of books cannot be sent using First Class  Large Envelope/Flat rates.  First-class PARCEL rate is usually the appropriate first-class rate for a book. 

• If you choose to send a book using First-Class postal rate, be sure to obscure the MEDIA MAIL printed on the PBS wrapper.

Related Links:

How much does it cost to mail a book?