Login issues with book nooks really depend on what the actual error message says, but I've had to deal with this enough times to develop a rough checklist. First thing I do is try logging in from a browser on my laptop instead of the app – if it works there, you know it's probably app-specific, and clearing the app cache or reinstalling usually fixes it. If it doesn't work anywhere, then the problem is account-level, not device.
For account-level stuff, the most common culprit is actually saved passwords causing trouble. I've found that typing the password manually, with all lowercase and checking for accidental spaces, solves about half my problems. If that fails, I hit 'forgot password' even if I'm sure I know it – sometimes sessions get corrupted on the server side, and the reset forces a clean slate. One weird glitch I've seen on a couple apps is regional settings interfering; if you've traveled or used a VPN, the app might think you're somewhere else and block login for security. Toggling VPN off or adjusting device location settings can unstick it.
Beyond that, it's about timing. If the servers are down, you're stuck waiting, but checking the platform's official Twitter or status page saves you a headache. I also sometimes switch from Wi-Fi to mobile data – maybe my home network is blocking a particular port the app uses for authentication. Last resort is contacting support, but you need to tell them exactly what you've tried; they appreciate the detail and usually respond faster.
Honestly, most login fails come from tiny, overlooked things like auto-capitalization on the first letter of a password field, or an app update that didn't install cleanly.