I have a question about something that's been on the back of my mind for a while: why the sporadic use of her/herself pronouns in D&D guidebooks? Is this a pointed attempt to be more inclusive? Wouldn't singular gender neutral they/themselves be more useful in the context of not knowing the gender of the presumed character? I don't hate it, I just want to know if there's a deeper purpose or source here.
It’s an inclusivity thing, yes. The singular they is generally disfavoured in tabletop RPG books because game rules frequently pertain to an individual interacting with a group (e.g., of monsters), so the singular they can create ambiguity with respect to whether the text is referring to the individual or the group. Thus, many games split the difference by alternating pronouns.
(It’s not the only approach that’s used, mind; a lot of games avoid the issue by phrasing rules in the second person, referring to the acting party as “you”. Dungeons & Dragons in particular hasn’t historically gone with this approach because it tends to come off as very informal, and D&D generally tries to go for a somewhat formal tone in its mechanical text.)