Yesterday I did some of the things to include about yourself in an agent letter. Here are some things NOT to include.
1. Feedback from non-industry professionals
Your friends and family will love your book. So will your grandchildren. In fact, most children enjoy being read to regardless of what is being read. The only time to break this rule is if your daughter/cousin/best friend is the book buyer for Tesco.
2. Feedback from industry professionals
Unless we're talking about the book buyer from Tesco then the answer is no. Why? Because the chances are that the feedback was essentially negative - or why else would you be looking for an agent? It doesn't matter how positive the rejection was, it was still a rejection. Don't include it because...
3. Nothing negative
You shouldn't have any negativity in your letter. I've seen statements such as: I've written many short stories and some have been accepted for publication. Why not simply write: I've had stories accepted for publication in XYZ magazine.
4. Endorsements from authors
Authors do, from time to time, show books to their agents. But they do the showing ie they contact their agent and say something like, read this book it's brilliant. Writing 'I have worked with author Jo Bloggs on the manuscript' isn't an endorsement from the author, especially if you paid the author to work on it. The same is true for literary consultancies/book doctors. If anything, it implies that you're not up to writing on your own; you'll need lots of handholding.
5. Personal stuff
I'm not talking just about sob stories (take me on or I'll kill myself) - but ordinary personal stuff. Is it relevant that you're married with two children? Almost certainly not, so don't include it. Does it matter if your family are behind you 100%? Nope. And yes, people do write this sort of thing - my first, unsuccessful, agent letter explained at embarrassing length how my novel Adultery for Beginners wasn't based on personal experience. I cringe in retrospect.
6. You've always wanted to be a writer
Um, most of us have. A lot of us write our first 'book' at primary school. And most writers are also avid readers, and that's part of the reason we write. We love books. This is almost a given, so not worth including.
7. You're published when you're not
If you say you've been published, the industry takes that to mean you've been published in physical book form by an established publishing company, or as an e-book by a well regarded e-publishing company. This is very easy to check on Amazon and any agent will know the publishers they consider to be established companies.
Claiming to be published when you're actually considered by the industry to be self published is a big no-no. Did you pay a contribution towards publication at any stage? Then it's considered that you're self published.
If you have self published (whether in physical book form or as an e-book) and sold respectable quantities - at least 1000 copies - then that's worth mentioning. If your garden shed is still full of stacks of books, then it's not.
Always get a friend to read over your biog and tell you honestly if it's too personal or full of irrelevant material. If in doubt, leave it out. It's better for it to be short - or non-existent - than for it to actively turn off an agent.