Eris Young likes to dream of alternative futures. Or pasts for that matter. Either direction is a rich resource for speculative writers, who adapt predictions and memories to reimagine our current world. 'Part of what I find the most exciting about my writing is this queer aspect to it,' says Young, whose short stories have appeared in places like 404 Ink's queer anthology We Were Always Here and Knight Errant Press's Quarrels With the Gender Binary. 'Sci-fi and fantasy give you so much opportunity to play with sex and gender and sexuality. If you're writing sci-fi and fantasy and you're not fucking gender, you're doing it wrong.'

But we're here to talk about non-fiction. For the past year, Young's business has been less with speculation and more with documenting the here and now. The result is their first full-length book, They/Them/Their, a holistic guide to genderqueer identities covering topics like health, law, language, and history.

'It's like a primer guide to being a non-binary person,' says Young, though they hope that the book reaches readers of all genders. 'It's aimed towards interested lay people – medical practitioners, educators – people who are encountering non-binary genderqueer people in their daily lives and want to understand where they're coming from a bit better.'

Part of Young's goal is to set down some standardised terminology for the trans and non-binary community. There was precious little around when they were coming out themself, and putting a name to their experiences mostly involved crowdsourcing answers from online forums. 'I mainly started exploring my gender identity on Tumblr,' they say. 'It was a super immersive environment full of genderqueer non-binary people, or people who fall under that umbrella. It was really good in a community-finding sense. I never would have found the validation that gives me without having found these people. But on the other hand, it means everything was extremely bootleg.'

Tumblr operated as a DIY manual, ranging from the nuances in the gender spectrum to symptoms like dysphoria and the existence of hormone therapy. In a chapter covering medicine, Young recounts the moment they first approached their family doctor about these treatments. 'It was quite a traumatic experience,' they say. 'I had to figure out what this was all about on my own based on information I found in blogs, forums, and message boards. I go to my GP and say I want to start hormone replacement therapy, and she had no idea. She'd never heard of it. I didn't know what to do. I left, and I had to start over.'

It fell to Young to source their own endocrinologist and, being based in the US at the time, the right medical insurance. That was after psyching themself up to seek hormone therapy in the first place, unsure if they would even qualify as deserving of it. 'I didn't really feel trans enough, and that's such a cliche,' they say. 'It's such a thing that a lot of non-binary genderqueer people, and even binary trans people, experience. Not feeling like the popular media portrayal of a medical transition is something that would work for you or like your identity is valid enough to demand it.'

Young's story will be familiar to many non-binary and trans people, who in the UK are usually required to live in their preferred gender for at least a year before they can undergo gender reassignment surgery. That's easier said than done when you don't identify as solely male or female. 'Gender presentation is really tricky and complex for genderqueer non-binary people,' says Young. 'There's perceived if not actual pressure to present as binary within these contexts, specifically as binary trans.' Young's book will give a wider picture of trans identities while showing the interconnectedness of the experiences covered by each chapter – be it the pressure to conform, how that affects mental health, or the cultural beliefs about gender that impact both.

Young noted several alternative models of gender while researching historical queerness, and the discovery of stereotypes in the Byzantine era has now sparked their speculative impulse – this time in a draft short story about magic eunuchs. 'Everyday people had opinions about eunuchs, in the same way that cis people have opinions about trans people that aren't always accurate,' says Young. 'Eunuchs were thought of as emotionally volatile, conniving. There's so much narrative potential in it. The idea of someone growing up with all of these conventions being put on them and how it affects how they grow up physically and think of themselves.'

This is where reality and speculation form two sides of the same imaginative coin. Unearthing facts inspires more questions, which allows storytellers like Young to construct possible answers. 'By writing about people who exist now and in the past, you give writers more fodder for speculating,' says Young. 'In the same way that speculative fiction allows you to postulate about how things could be different, non-fiction gives you a way to give voice to people who don't have voices now.'

They/Them/Their: A Guide to Nonbinary and Genderqueer Identities is out Thu 19 Sep via Jessica Kingsley Publishers.