I just started using the app and I have created brand new campaigns for my KDP books. I have one auto campaign and one exact keyword/ASIN campaign. Do you suggest to set these campaigns with Fixed,...

You can choose Dynamic bids – down only for Auto campaign so when you click on your ad it is less likely to convert to a sale, they will charge you less than your max bid, thus you can save money on this. For Manual Campaigns, you can choose  Dynamic bids – up & down bidding. This option will raise your bids or lower your bids depending on the data if the clicks will lead to a sale. This will have a high chance of getting more conversions.