yeah, you should because I think if you dont read the previous books, you wont understand what's happening in the "how to steal a dragon's sword". By the way, I have been searching long for the eBook of HTTYD, and i found the first 5 books here link I hope i helped someone :)

Personally, I like to read books in order. Always. I don't know too much about the HTTYD book series, but I think it would help if you read the others.

I think you should read in order that's what I did.
oh, and How to Steal a Dragons Sword a 4 part story this one's the 1st