I think when you choose to read The Assassin’s Blade makes a significant impact on your level of enjoyment. I waited until I had completed the Throne of Glass series before reading as I was worried about spoilers, but that may have been a mistake.

I could not stand Celaena, so after having read seven books of her evolving into a more tolerable Aelin, it was insufferable to go back to reading about her as Celaena. She’s the same shortsighted, immature Celaena from Throne of Glass and Crown of Midnight. Her actions continuously put others in danger. While she is often working to help others, it’s usually because she’s been forced into it by her short temper and unreliability. Without the relief of alternating POVs like the rest of the series, it was just too much Celaena.

These novellas add to the backstory and add more colour to some of the supporting characters that show up later in the series, so I would recommend reading them. However, I’d recommend reading them second, after Throne of Glass. If you’ve read the first book and are planning to continue the series, The Assassin’s Blade would fit well here, before reading Crown of Midnight. There are no spoilers – all of the stories take place before Celaena goes to Endovier – and at this point, you’ll still be used to dealing with immature Celaena.