Just received first request for my manuscript - question

As the title states, I just received my first request for my manuscript (full at that). My book is part of a series but I did not mention that in my original query letter. I'm wondering if I should mention that I have another book that's part of the series and been written/ready to go for in case they like my book that I'm sending now or should I just wait until they decide to sign me (wishful thinking)?

As I've stated in other threads before, my book is a short Humor book with a length of 4,000 words that would need illustrations. The reason I would like to mention my other book is because it has a length of 10,000 words and I'm afraid that the first book may be considered too short (although I understand alerting them of a second book wouldn't change that). Either way I'm going to work now and am planning on sending the agency what they requested later tonight when I get home. Any input is appreciated. Thanks.

Thank you guys. I forgot to mention that the word count was included on the original query so the agent is aware. I'm just conscious of the word count after a lifetime of hearing about how much size matters

Thank you very much. I researched this agent and she apparently requests manuscripts quite a bit so I'm not as excited as before but regardless it's definitely good to get one request under the belt. If anything good comes of it, I'll let you all know.

Again, thank you for all of your input. Now we play the waiting game haha. In the meantime I'm excited because I just found another agent who seems perfect for my book (it really fits the profile of what he's looking for) so I'm going to wait until Monday to query him as I've heard it's not good to query agents during the weekend because it can get lost in the crowd of others.