I'm following the Houdini Foundations Book [www.sidefx.com]. I am at the chapter about shading and rendering, where it is described how to load a normal map in the Principled shader and adjust its intensity. When I tried it, I found that the viewport doesn't seem to follow the intensity set in the shader. The image below illustrates the difference between viewport and render.

Is there a way to adjust the intensity with which normal maps are applied in the viewport? I did some search in the docs and the forums, but couldn't find anything.

I don't think it is implemented yet.

I've just learned a way of having this functionality; however, it requires editing the shader. These are the steps:

(Edit: For the sake of brevity, and for those who already know the procedure, the workaround is to tag the /Bump & Normals/Base/Effect Scale (baseNormal_scale) parameter in the Principled Shader with the /OpenGL/Normal Scale (ogl_normalmap_scale) tag.)

1. In the Parameters editor of the Principled Shader, click the Gear icon and pick Type properties.
2. From the Edit Operator Type Properties window that appears, save a copy of the shader to a local library, by setting its new path (e.g., $HIP/hda/OPlibVop.hda) and pressing Apply. This will require unlocking the node.
3. In the Parameters tab of the same window, below Existing Parameters, select /Bump & Normals/Base/Effect Scale (baseNormal_scale).
4. Under the Parameter Description section, find the Tags list and add a Built-in one, /OpenGL/Normal Scale.
5. Press Accept to save the changes, and lock the shader node.

Then, in the shader parameters, changing the Effect Scale under Bump & Normals/Base varies the intensity of the normal map on the viewport.