The Friends bought books for us, where do I put the expenditure?

If the Friends or another group, give the library cash, and the library controls how it is spent, then the amount is "Other Revenue."  When the library spends the cash, the expenditure of that cash is reported with other expenditures.  If the Friends keep control of the cash, and instead present the library with books, or other items, then the library does not record either revenue or expenditure.

Libraries should not record the worth of donated items in the statistical report.  Basically, the statistical report is a measure of inputs and outputs, with the general question of "what amount of resources does it take in a normal year for the library to do what it does."  The library doesn't control the actions or budget of a separate group, and gifts are not predicatable like normal revenue.

There is a correlation between expenditures on books and the collection count, but it is not exact as the library adds a number of donations, and weeds a number of volumes.