[cctalk] Good computer books for 1960's computers

Having finished a 20 (21*) bit word biased computer with 18 bit memory addressing and binary and execess-3 math, I need to write soft ware this beast. All I have is a serial bootstrap loader,and a cross-assembler in K&R C. Bare routines for Console and DISC (SD card) are being written. Are there any good(used) books on languages and operating systems that don't cost a arm and leg. Kuth,BCPL and FORTH are already covered? This is what I consider a 1960's style computer other than IBM 360 so modern software is out. Ben. * Parts and revised PCB's expected to arrive by then.