Dear Mr/Ms. Sydney,
Psychiatry has never been so dangerous. [At first blush this looks like it would be a good hook, but it's actually not - don't use it. Go straight to the next para.]
Alpha Slip is a novel of paranoia, conspiracy and fractured realities. [Good - concise, clear, descriptive.] In 2065, Detroit is a broken city where the police can be bought and surveillance is omnipresent. The only place left to speak freely is in your head: rebellious youth use hacked psychiatric tools to plug into each other’s minds and share dreams of sex and false enlightenment. [Again, good. Interesting premise, and you've set place and time without any fuss.]
Professor William Vice is the pioneer of this dream-melding technology. He comes out of unhappy retirement to help his old students treat Buckley, a child soldier returned from the civil war on Mars. But when one of his students is found murdered and Vice is accused, he finds himself on the run from the police, Buckley, and his mysterious military backers. [I'd stop the description here and not use the next para - you've left us dangling with this one, and that's all you need.]
To make things worse, Vice is suffering delusions of his own. His greatest talent – his skill in melding and slipping through patient’s dreams – may also be his undoing. The meld implants in his head are old and failing, and picking reality from hallucination may be the greatest challenge of all.
Alpha Slip is [a] completed 110,000 word manuscript, and would appeal to fans of Richard Morgan, William Gibson and Philip. K. Dick. [Good - word count and references.] It is my fourth completed novel [don't say this unless the first three were published], and I sincerely feel it is ready for publication. [Don't say this either - if you don't think it's ready for publication, you shouldn't send it to me, so saying this just makes me think that maybe you're not sure.] I am querying you specifically because of your interest in aggressive and edgy spec fiction - I think Alpha Slip tackles a number of provocative issues, such as the enslavement of child soldiers and the effects of a surveillance state, without losing sight of its classic sci-fi roots. [Good]
Thankyou for your consideration.
Overall: a very good letter.
What you haven't mentioned: why you're writing this type of story. Because spec fiction has a passionate readership and equally passionate writers, with lots of subgenres, so for this genre - and for romance - it can be good to mention why you've chosen your particular subgenre. It will help the agent/publisher envisage where it fits. But it's a nice-to-have, not a must-have.