Mastering reports isn't easy—you need to master all the reporting features that are available on the platform (and this book will help you achieve those skills), and also be able to properly translate business needs into report specifics (and it can be all but uncommon that your business needs don't fit with the available reporting features). This is where experience will help you out.
For a detailed list of limits and considerations when it comes to reporting, take a look at Salesforce Help at https://help.salesforce.com/articleView?id=rd_reports_limits.htm&type=5. The common troubleshooting FAQs at https://help.salesforce.com/articleView?id=rd_reports_troubleshoot.htm&type=5 will also help you with this.
