Find out exactly what's available to you thru your library. Mine has Overdrive, Hoopla, and something else that has classics out of copyright (can't recall the name). Overdrive works on a one-book, one-checkout system (have to wait for your turn), but you are limited to a set number of checkouts at one time. Read them and return them, or choose the 7 day checkout instead of 14 day, to clear room to checkout more. Hoopla has unlimited copies available (never wait), but limits your total checkout number per month. They have different selections.
If you like the classics, the ones out of copyright can be free, and should be better than potluck on Amazon's free list. They can also be available on Amazon, though probably not in the top 100. Just search for titles and see if there is a free version. There will be versions that are NOT free; anyone is allowed to create a nicely typeset new version and charge if they wish. Don't select these by mistake.
As for Amazon's top 100 free list - I check it every few days, as things may pop on and off again quickly, say a short term free offer of an author's new sequel. If something looks interesting, I read the reviews - people will call out poor spelling and illogical storylines, so you have some idea if it's a bad choice. If you have a favorite genre you are looking for more of, select that genre on the left hand side menu - often you will now see the top 100 free books in that genre, which would normally be further down the main list, so you see more titles.
Then there's lending thru Amazon - I haven't tried it yet, so don't know how it works, but explore the idea.
I tend to get a free book from Amazon when I see it - it may not be there later ( or may not be free). Once I own it, it can sit until I'm looking for something to read. Books I checkout from services I try to read immediately, or I'll forget to finish it in time. On my old kindle (not a Fire) I only used WiFi when downloading, so as long as there was no internet connection I could continue to read Overdrive ebooks beyond the due date - as soon as an internet connection was established, the DRM software world lock up the overdue book. Essentially, it didn't get the message to lock the book until it had an internet connection. That trick doesn't work so well on my kindle Fire, since I use it as a tablet and not just an ereader.