Sadly, besides the official User Guide, there's very little. The old SynthMaker Wiki has a few additional tutorials (
downloadable here - NB: very big zip file!), but not a huge amount. An awful lot was lost along with the SynthMaker forum, I think, including many of the best DSP/ASM 'gurus'.)
Before even thinking of ASM, definitely try to master DSP first. As well as being a whole lot easier to learn, DSP code translates directly into ASM, which you can access from a DSP primitive's 'C' output. Working in ASM often begins with making a prototype in DSP, and sometimes there isn't anything that could be optimised any further. ASM can do a few things that are impossible with DSP code, but you're best off learning the things that are equivalent to begin with.