When querying, should I mention the three other never-to-see-the-light-of-day manuscripts I've written or not? I feel their existence shows I've been working hard on my writing for some time, but they're not published (nor should they ever be). Also, if I do mention them, what should I call them? Unpublished novels? Blackmail material?
First of all: well done on completing three full-length manuscripts before writing the one you intend to submit. Consider them your apprenticeship. Also evidence of your patience. So, yes, for those reasons you should mention them - and you can do it in a humorous way (as your suggestion of 'blackmail material' indicates that you have a sense of humour) or any way you like. I personally like a touch of humour in a submission letter, done appropriately.
However, you can also just say that you have three manuscripts tucked away in a bottom drawer (hint: this is what you can call then - 'bottom drawer' being a phrase that publishing types like to use) that you never intend to submit to anyone - just to allay an agent/publisher's fear that you're about to send him or her three extra manuscripts - and that you feel that the one you're submitting is the beneficiary of everything you've learned on the previous three. Because that will probably be the truth. And agents/publishers do like to see evidence that a writer hasn't just finished the first draft of a first novel and sent it off that very same day.
Having said all that, if you don't mention them it's not going to affect your chances of being taken seriously. We always read the submission that's before it on its merits, regardless of what's come before. So go forth, submit and good luck.
First of all: well done on completing three full-length manuscripts before writing the one you intend to submit. Consider them your apprenticeship. Also evidence of your patience. So, yes, for those reasons you should mention them - and you can do it in a humorous way (as your suggestion of 'blackmail material' indicates that you have a sense of humour) or any way you like. I personally like a touch of humour in a submission letter, done appropriately.
However, you can also just say that you have three manuscripts tucked away in a bottom drawer (hint: this is what you can call then - 'bottom drawer' being a phrase that publishing types like to use) that you never intend to submit to anyone - just to allay an agent/publisher's fear that you're about to send him or her three extra manuscripts - and that you feel that the one you're submitting is the beneficiary of everything you've learned on the previous three. Because that will probably be the truth. And agents/publishers do like to see evidence that a writer hasn't just finished the first draft of a first novel and sent it off that very same day.
Having said all that, if you don't mention them it's not going to affect your chances of being taken seriously. We always read the submission that's before it on its merits, regardless of what's come before. So go forth, submit and good luck.