advice needed

An unsolicited manuscript is something they haven't asked for. A preliminary letter is a short (1-2 pages) introduction of your novel and, to a lesser degree, you (ie, stuff related to writing like when and where you've had stuff published), asking them if they would like to read your manuscript. If they write back and say yes, and you send them your manuscript, it's then 'solicited', because they've asked you to send it.

...and beware...many of them won't accept unsolicited manuscripts...and won't thank you for sending them one.

Writers and Artists Yearbook good in this regard, although the number of entries that say not taking new work or unsolicited mss grows every year. You waste a lot of postage sending your crime novel to people who only take romantic fiction without WaAY, so it is worth the tenner it costs.

..and a good idea to update it every year, too...