[Quoth]

If the ePub (created by Calibre, no DRM) is put:
1)In "Digital Editions": It has Table of Contents via Menu (like a PDF menu). Zoom changes font size. Fonts are sensibly used. Esp, body font. No Font or Style in Menu. All Covers and Images work. Justification & Line spacing is according to original document, (left) and can't be changed. So I'll use a fully justified source. This is nearly fine.

2)If in "My Books" the Menu offers Fonts & Styles, but body font face is locked to YaHei. Size changes. Only some books have covers. Internal images work. Linespacing & Justification settings work. No TOC
3) Any place other than My Books or Digital, the fontface is locked to Courier instead, otherwise like (2). No TOC

As there is no cursor, links in books or contents don't work! Links work even on my old Keyboard no touch Kindle DXG. You can do a Plot Your Own Adventure book in Mobi or ePub format using links. There is even a no go past this page tag by page turning.

If Mobi is used in My Books:
No covers or images at all. Headings are same size as body. Default is Courier & 1.4 LS, but you can change it to DejaVu Serif, and any size / spacing.

If Mobi is used in arbitrary folder:
No covers or images at all. Headings are same size as body. Default is MS YaHei face & 1.4 LS, but you can change it to DejaVu Serif, and any size / spacing.

FB2 Format behaves like Mobi, except header size preserved, but no page breaks.

Conclusion: Useable if ePub books are put in "Digital Editions" and source has justification & line spacing desired.

Deadly uselessly slow for image based PDFs (scans).

Unlike Kobo Aura H20 HD /Kindle PaperWhite, zero annotation. Kobo is best for proofing as annotations have highlighted context and per book when imported back via Calibre.

Calibre doesn't detect it as a Binatone, (which it has), but variously Pocketbook-> The Book -> LibreAir as you disable. I used the USB IDs in the Calibre User Device Plugin, which works to detect what is already in the "Digital Editions" folder. However it "sends" fresh books to udisk fake "root" instead of the "Digital Editions" folder. Even when that's entered in the plugin config, maybe it wants /Digital Editions/ or "Digital Editions"?