Function not successfully reading sensor registers
Wire.requestFrom(register_address, numBytes);
  byte c;
  Serial.println(Wire.read());
  while (Wire.available()) {
    c = Wire.read();
  }


This has a number of problems. For one thing your (I presume) debugging print consumes the data from Wire.read, so when you go to get it into c, you have already printed it. In other words, you are reading byte #1 and printing it, but using byte #2. This seems an odd thing to do.

Next, what is this doing?

  while (Wire.available()) {
    c = Wire.read();
  }


You are reading an indefinite number of bytes, and retaining the last one. Why?

Possible rework:

byte fp::readRegister(byte register_address, int numBytes) {
  Wire.requestFrom(register_address, numBytes);
  byte c = Wire.read()
  Serial.println(c);
  Serial.print("0x");
  Serial.print(register_address, HEX);
  Serial.print("\t");
  Serial.print(c);
  Serial.print("\t");
  Serial.println(millis());
  return c;
}





  Here is the Git repo with the rest of the code: https://github.com/fpdotmonkey/fp_accel


From your linked code:

  byte xlm = readRegister(fp::OUT_X_L_M, 1);
  byte xhm = readRegister(fp::OUT_X_H_M, 1);
  byte ylm = readRegister(fp::OUT_Y_L_M, 1);
  byte yhm = readRegister(fp::OUT_Y_H_M, 1);
  byte zlm = readRegister(fp::OUT_Z_L_M, 1);
  byte zhm = readRegister(fp::OUT_Z_H_M, 1);


You have the wrong end of the stick here.

  Wire.requestFrom(register_address, numBytes);


You request from the I2C address - that is the device address. The register is something else.

Judging by the datasheet you first have to send the register address, and then request the data. Something along the lines of:

byte fp::readRegister(byte register_address, int numBytes) {
  Wire.beginTransmission (DEVICE_ADDRESS);
  Wire.write (register_address);
  Wire.endTransmission (false);  // repeated start
  Wire.requestFrom(DEVICE_ADDRESS, 1);
  byte c = Wire.read()
  Serial.println(c);
  Serial.print("0x");
  Serial.print(register_address, HEX);
  Serial.print("\t");
  Serial.print(c);
  Serial.print("\t");
  Serial.println(millis());
  return c;
}


Similarly for writing to a register:

void fp::writeRegister(byte register_address, byte val) {
  Wire.beginTransmission(DEVICE_ADDRESS);
  Wire.write (register_address);
  Wire.write(val);
  Wire.endTransmission();
}


This is untested because I don't have the device here, but I know that what you had won't work. To check the actual device address run the I2C scanner I mentioned before.