For me that really depends on what I am dealing with. Most of the time the library source code is good enough. Sometimes - I am a C# developer - I will have to

For me that really depends on what I am dealing with. Most of the time the library source code is good enough. Sometimes - I am a C# developer - I will have to have a look at the .NET runtime specification. Sometimes I will have to learn details of Windows. In some cases I will even look into Intel's reference manuals to understand what is happening. I would say I stop at the level where I don't expect the layer below to be relevant to what I am actually dealing with.