Some parts of fp64lib are based on the 32-bit floating point routines, which is published on under a single unified license. This so-called modified Berkeley license is intented to be compatible with most Free Software licenses like the GPL, yet impose as little restrictions for the use of the library in closed-source commercial applications as possible. To make things simple, the same license applies to parts that are developed by me.
The easiest way is to include fp64lib via the builtin Library Manager of the Arduino IDE. If you are not familiar with the Library Manager, see here for instructions.
You can also download the lastest version of the library here, that can be directly included into your Arduino IDE:
If you want to explore the source code, go and check the github repository:
Please use also the github page to report any issues/bugs.