even the shaders are in HLSL, in monogame this will work in a WindowsGL Project? GLSL was not to be for openGL?
and if I can use HLSL for WIndowsGL project, then every book that I read of XNA Shaders will serve for Monogame shaders too?

Yes it will work because it used crossplaform utility called MGFX so your shaders compilable for both DirectX and OpenGL platforms(with some minor changes you must do). Shader model is close to HLSL 3.0 as much as possible(as its predecessor XNA).