I'm at the laundromat. I have a 20mb epub in my Dropbox. I try to open it in the Dropbox app. It goes, I can't open this, but here's a button to upload it to Google Play Books. That's convenient, I think; I tap it. It spends something like twenty minutes downloading the epub to the phone over a cell connection, then another twenty minutes uploading it to Google Play Books. Eventually it finishes. I open Google Play Books. The book is not there. Nor is it in Files. There is no error message.
This story has no moral.

*Later, trying again from the computer*

Google what the fuck does "Processing" mean. What does it mean to process a book. I just want you to sync it to my phone and open it. I mean it's already on my phone but you won't open it on my phone unless you first sync it from my phone to your server back to my phone so I'm trying to be accommodating.

If I click on the book all it does is open a new tab containing this help page https://support.google.com/googleplay/answer/185545?visit_id=638955591931852971-3016000300&rd=2

Has anyone ever seen Google Play Books do this, and if it does, is there a way to get an error message? I am in contact with¹ the publisher but there is not enough information to tell the publisher what is wrong.

¹ Married to

Alternately I am now looking for recommendations for a good basic epub reader for Android. All I want is for it to have text search, have a good upper range of font sizes, and not display "noise" such as page numbers or the current time while I am reading. I have never needed anything besides Google Play Books before this.
@mcc ReadEra is basic and can turn off all indicators, I just checked and turned off the last one and on mine (so thanks for prompting me to look!) It also goes very big in font sizes and has text search
@paulsilver hmm… downloaded this one… unfortunately it appears it only has two options for page margins, "none" and "comically huge"…

@mcc Ah, sorry, if I'd seen that I would have mentioned it. All the epubs I've read in ReadEra have had decently small/standard margins so I hadn't tripped over the limitation.

I'm glad you got the original problem sorted

@paulsilver my phone screen is an unusual shape, maybe on a bigger phone the margins look better