Device or app? I've found that pretty much all technical books look terrible on Kindle devices - the screens are too small, the technical content (source) usually formats poorly, and diagrams are often screwed up. That's not unique to Manning.

I have a lot of Manning books that I've purchased directly from their site and/or from Amazon. I generally read them in the Kindle app (iOS, lt, web) or in iBooks. The only Kindle device that I've comfortably read any technical books on is the old Kindle DX, and that was just because the screen was relatively large.

Technical content and smaller e-reader devices will never be a good fit. Occasionally it's tolerable, usually it's not.

I'm just fine reading them on an iPad, though. I typically download the epub from their site and upload it to Google Play Books, so I can access it from all my devices.

The latest book "C#, In Depth", Fourth Edition is really readable, even the code, on the iPad (9.7"). I even occasionally read some on my Android phone and while the code samples fold on themselves, they still make sense at atleast (nothing getting cut off by invisible scrolling or anything).

I don't know how people would expect to read a tech book on a 6" screen of a Kindle, there's a reason print versions of tech books aren't the same size of a mass market genre paperback.

As an author with them I have complained about this issue many times. I am sorry to say I have not made much headway. I believe if the right people knew, they would do the right thing, so I will continue to try.