I am looking for clear and easy-to-understand references about (modern) GPU architectures, that would for example explain (in generic terms) the flow of data, the way problems such as synchronization issues are handled. etc.

Most often the documents I find are:

- Presentation slides

- Technical specifications

Since those are quite hard to understand, I am looking for explanation with sentences written in "regular" English.

So far the closest thing to what I need are probably the posts "A trip through the Graphics Pipeline" by Fabian Giesen.

Are there any good books with the same level of quality and clarity, dealing with GPU architecture?

You mean, something like this?

I have found this article on gl buffer transfer provides a really good foundation for memory management in gpu drivers from the perspective of an application programmer. The textbook it's from is definitely a bit old (2012 I think), but it may still be relevant.

If you're comfortable with reading through hardware documentation, then AMD have some on their website.

http://developer.amd.com/resources/documentation-articles/developer-guides-manuals/

I think it's the programming guides under Open GPU Documentation that you want to look at.

EDIT: Just reread the OP where you say you want easiest to digest material. The above link is still something you could bookmark if you ever want to go back to it :)

Here is a presentation lecture on the matter, from an ATi/AMD engineer: