Can I include a header file into a new header file? Can I include a header file into a new header file that I created? I put #include &lt;MCP23S17.h&gt;. I want to create a another header file.
Yes, a header can include another header.  But if you are using the Arduino IDE and the headers aren't in the same folder, then you'll also need an #include line for it in the .ino file.