Welcome along, there are plenty of 900 and air cooled devotees here so it's a great source of help and information.
A Haynes manual is a good start as there isn't a great deal of difference between years and even many parts are inter-changeable between the capacities too.
I have one of the first editions covering model years '91-'96 but it's been fine for my 1997 900.
https://www.ebay.co.uk/itm/Haynes-Wo...QAAOSw1Pxe0rol
Although there is a later edition now which covers the injected models up to 2004 also.
https://www.ebay.co.uk/itm/Ducati-60...6clp%3D2334524