I don't read to often but found out that it's cheaper to buy the books I do read on Kindle, is the app on phones good as I don't really think paying out for Kindle is worth it for me at the moment.
Are you sure you would read books on your phone on tablet? Ink display is much better, and you can get a used kindle for very cheap.
I can't tell you whether the app is good as I've never used it, but there are other apps available, google books is the one i tried on Android and seems decent, but haven't actually read any books on it because phone screen isn't as comfortable for me, even if it's oled.
I can't gush enough about how much I prefer reading on the Kindle. But the app is okay, in my country I can't buy from the app, I think because Google doesn't allow in-app purchases that they don't get their 30% cut from.
You also need to download the dictionary on its own. It's similar to a kindle though and much faster then a kindle. Also great for quick reads say on short bus/train trips. Syncs quite alright as well.
I'd say download it imo, then get a few of the free or cheap books (some Agatha Christie novels are like a dollar!) Try it out and see if it works for you.