You can convert $\displaystyle y''+Py'+Qy=0$ into its normal form $\displaystyle v''+Iv=0$ by letting $\displaystyle y=v\,\exp\left(-1/2\int Pdx\right)$
Got two references for you: "Intermediate Differential Equations" by Rainville and Handbook of Differential Equations by Zwillinger. Try and find them in the library.