Can you recommend me a guide or book for making apps focused on people with vision problems? Some kind of accessibility guidelines.
I know there are some guidelines in the iOS/Android documentation, but I'm looking for something more detailed.
As mentioned by @katriel, the Web Content Accessibility Guidelines (WCAG) are the official W3C specs on how to create accessible applications (whether web or mobile).
Most country laws are based on Version 2.0 (at least for now) but that will be changing. The European Union is typically faster than the US in updating their laws so the EU's EN 301 549 will probably be updated before the US's Section 508.
You will have to decide (or you company will have to decide) whether to follow WCAG 2.0 or 2.1. Focusing on 2.1 now may make your life easier in the future.
The W3C also has a document relevant to your question: "Mobile Accessibility: How WCAG 2.0 and Other W3C/WAI Guidelines Apply to Mobile".
There are currently some excellent books on digital accessibility in general as well as classes via Treehouse or Coursera, and web-based applications may fall under various laws depending on country (currently, in the US, there are lawsuits under the Americans with Disabilities Act pending about website accessibility for example). But there is no one standard that is similar to the WCAG standards (link to WCAG 2.0 - I'm linking to 2.0 because that's also what the US government's Section 508/accessibility law is based on), that covers mobile apps specifically.
That being said, please try to review WCAG 2.0, and in terms of implementation, read up on the accessibility APIs. This guide to testing and debugging in iOS Simulator is also detailed. Hope that helps!