There are more than a few books and tutorials on how to write parsers and compilers and PL theory, but I know of none on how to go about actually designing a language from scratch. Perhaps most new languages can be considered slight variations on earlier languages, so there's not much point when you're almost certainly using one of ~6 styles with slight modifications? I got started on my own little project by writing "ideal" programs and then working backwards and defining the syntax and vocabulary, then writing and reworking again ad nauseam.