Many sites offer audio book downloads for Sansa mp4 players. Most audio books are sold in mp3 format but you do not have to download mp4 files. Many programs can be downloaded online that will convert the files. Find the audio book that you want to download and make sure to save it to a place that you can easily access. Generally the desktop is a safe place.
Some computers will also automatically download items to the "Downloads" folder on your computer. Usually this folder is found in the "My Documents" folder. Do not download books from sites that look questionable. Generally, .net sites are less reputable than .com sites--although this is not always the case. If the site offers best-selling books for free or much cheaper than usual, you should probably not do business with them.
Once you have the files downloaded to the computer, you must convert the files to mp4 format so that they can play on your mp4 player. Go online and find a program that will convert mp3 to mp4 files. Many free programs do this, so there is no need to pay for conversion software unless you plan to download a lot of audio books. If this is the case then you will probably run out of the free conversions that many software programs allow before charging.
It is common to see programs that offer a certain number of conversions for free, before they start charging. As long as you still can do so, take advantage of the free conversion offers.
Set your player to either "MPT transfers", if you use programs like Widows Media Player or Napster, or to "MSC transfers" if you want to transfer the files over, like moving files from one place on the computer to another. Move the files onto the Sansa and make sure that you have the files named as a file that you will recognize and that they are in the right order. Typically, using the MTP transfer style is easiest for a large block of files like audio books.