[RPG] Legality of publishing a book based on a D&D game

In general in play they were ignored or just treated as an abstract language with no further comment.

As to where they came from, here's an answer from Gary Gygax on Dragonsfoot!

As D&D was being quantified and qualified by the publication of the supplemental rules booklets. I decided that Thieves' cant should not be the only secret language. Thus alignment languages come into play, the rational [sic] being they were akin to Hebrew for Jewish and Latin for Roman Catholic persons.

I have since regretted the addition, as the non-cleric user would have only a limited vocabulary, and little cound [sic] be conveyed or understoon [sic] by the use of an alignment language between non-clerical users.

If the DMs would have restricted the use of alignment languages--done mainly because I insisted on that as I should have--then the concept is vaible [sic]. In my view the secret societies of alignment would be pantheonic, known to the clerics of that belief system and special orders of laity only. The ordinary faithful would know only a few words, more or less for recognition.

In other words, it was supposed to be more like religious languages, but wasn't really well thought through. It disappeared in Second Edition and was not missed.

The simplest way to reference monsters is to follow the lead of WotC published adventures and not include page numbers at all. At the beginning of your publication, explain your writing convention ("monsters named in bold print can be found in either the SRD or one of the core rule books of the 5th edition of the world's most popular pen and paper RPG"), and then trust your readers to know how to find them. For better or worse, 5e DMs have been trained not to expect page references after every monster name, so it's likely that none will even notice that your product doesn't have them.