Hello Gerry man. Howard M. Berlin's "Design of Active Filters: With Experiments" is a good book. It will give you a good understanding of filters and expand your knowledge of clever ways of using them. It covers analog filters based on the 741. These days other opamps such as the TLO72 will yield even better results. Knowing this will also give you a better understanding of using digital filters. Keep in mind that filters are't just tone controls, and can be used for envelope shaping, amplifier control, even control of other filters, or for that matter even control of lights. From there, your imagination can take over. It is remarkable what effects you can make with creative use of filters. Have fun.