Using the Arduino environement, as I understand it, you cannot access your own library from another of your own libraries. There is no way to add paths, so there is simply no way for the compiler to find the code. That makes it hard to write libraries that use code in another of your libraries. My web research indicates this has been a problem for years but to my knowledge has not been solved. I suspect there are difficulties in the implementation details or perhaps a desire to keep the system simple at the expense of capability.
Of course, you can always cut and paste code into each new library, but that's exceedingly sub-optimal. You can also write one huge library with all of your code in one pair of .h and .cpp files. That's also not very satisfactory, but I've done it on occasion.
There is a work around, however, for using standard Arduino libraries in your own library that you're placing in your sketchbook/libraries directory. Since sketches include paths to the standard library locations, and link the standard library code, you can include the header file for the standard library of interest in your sketch. Below that, also in your sketch, include your own library header file. The standard library will then become available to your library as well as to your sketch.
