Monergism.com has a lot of free ebooks and audio. One excellent resource that they have available is "Basics of the Reformed Faith" by Dr. Kim Riddlebarger. Dr. Riddlebarger is a pastor and a regular host on the White Horse Inn radio program. You can get this resource from this page on the Monergism website.
The file you will download for this eBook is a .mobi file which can be used on a Kindle. The best method for getting these types of files on your Kindle is to download the file and then email it to your @kindle.com email address. You can find this address by going to "Personal Document Settings" in your Amazon account. Attach the downloaded file in an email to your @kindle.com address and it will be sent to you Kindle device.