Apps for offline manga reading
Well there are a couple of resources but this review page [alternate webarchive link because it was down] might interest you pertaining to certain uses. I'll just leave it as an answer here instead of the comments.
The author titles the post :

This is How You Can Read Manga Legally in 2013

While not quite 2013, it has still many relevant links:
First the author mentions E-manga

Where you can read it: It’s a worldwide service.
How you can read it: Bunch of ways: you can download it in 7 formats (PRC, MOBI, EPUB, PDF, CBZ, Apple EPUB, Kobo EPUB), for a bunch of devices, though it depends for some: PC, iPhone, iPad, iPod, Kindle, Kindle Fire, Android, Nook 6”, Nook Color, Kobo 6”, Kobo Tablets, Sony Readers, Linux, Mac, Windows, and Palm OS, Windows Mobile, Motorola Xoom, Abode Digital Editions, Blackberry, Bebook, Aluratek Libre Pro, Cybook Opus, Digital Reader 1000, EZ Reader by Astak, Hanlin, iLiad, iRiver Story.

The author further provides their social media, email and about links in the review page:

Anything Extra: Registration here
Email: customerservice@emanga.com
Social Media: Twitter, Facebook

He also does a mini-review or commentary of that particular site that he lists which I found quite nice:

They have just about every type of manga you want: Shoujo, Shounen, Seinen, Josei, Yaoi, Yuri, Hentai, the works. They are pretty pricey though, ranging from the $9.99 range or higher for a good portion of their titles. As far as I know, most people don’t like to spend that much for physical books; they’ll probably just laugh at seeing the prices here. Otherwise, they’re one of the few publishers that actually allows you to have files for you to keep.

He goes on in this format, mentioning other sites:

MangaReborn
Gen Manga
Viz Media
Weekly Shonen Jump
Netcomics
Kodansha USA
Yen Press
Vertical Inc
Dark Horse
Seven Seas
SuBLime Manga
Rental
Amimaru Manga
J-Comi
Crunchyroll

While the sites are directly linked here, if you need more information other than where to buy electronically scanned manga, like the reviews for each site, support the author by going to the site.