Hi there Lilja! My name is***** am a computer professional with over 15 years of experience with both Mac and Windows computers, phones, tablets, e-readers, and printers. It would be my pleasure to assist you with this. You can safely delete the books that you have already read from your Kindle, without fear of losing them. The books that you have purchased from Amazon for your Kindle will always be available at the Amazon website under My CONTENT if you should wish to download them again at some point in the future. To test this, try deleting a few books. Next, click the link below:
Enter the Amazon account information ( the email address and the password) associated with your Kindle. After you login you should see 3 tabs (Content, Devices, Settings). On the CONTENT tab you will see all of your Kindle book purchases. You can sort them by title, author or purchase date. Please locate the books that you have deleted. To re-download a deleted book to your Kindle, locate it in the list, select it by checking it and then click on the ACTIONS column. Choose deliver as the action. Select your device from the Device list. If you have more than one Kindle, make sure that you choose the Kindle that you are actively using. Don't select the Kindle Cloud reader unless you have downloaded the Kindle Cloud Reader APP to a computer or device other than your Kindle and wish to read it there.
The apps that you have purchased from Amazon will continue to be available to download from the Amazon App store as well, without requiring you to repurchase them.
Please give this a try and let me know how it works for you. It will not be necessary for you to save your books to a card, or anywhere else. If this solves the problem, would you mind taking a moment to rate my service? I'd really appreciate it! If you need more help,or have more questions, please let me know, and please tell me exact model of your Kindle. If you're not sure, please look for a serial number. If you can't find it on the Kindle, you can find it on the Manage Your Kindle page of your Amazon account (in the Manage Your Devices section). That will help me to provide better answers if you still have questions. Thank you!