It’s available from Amazon here:
Waterstones here: https://www.waterstones.com/book/100-years-of-leeds-united/daniel-chapman/9781785784309
…or in branches — they’ll order it in if it’s not in yours.
Any good local bookshop will be able to get you a copy, and it’s in the club shop.
You can get copies signed and sent by me through the TSB shop here: https://www.thesquareball.net/shop/100-years-of-leeds-united-hardback/
If you want it quick and cheap go with Amazon, signed and slow go with TSB.
If you’re outside the UK, check your local online or offline bookshops; it’s on the shelves in Easons in Dublin (next to a book about Solskjaer, so if you could cover that up while you’re there, that’d be great); Booktopia have it in Australia: https://www.booktopia.com.au/100-years-of-leeds-united-daniel-chapman/prod9781785784309.html and in Sweden from Adilibris and others: https://www.adlibris.com/se/bok/100-years-of-leeds-united-9781785784309