Agree with ScreamPillar about checking out the Writers yearbook - most larger public libraries should have a copy.
Also use the internet to do your research, find out which publishers publish in the equestrine field, it's no good contacting a chick lit publisher.
Most will request either a synopsis ( usually no longer than 1000 words at most ) of the book or a synopsis and sample chapters. They may also ask you to tell them about competing texts, I had to write a report / literature review of the triathlon book market. I was also asked to write a report on who it would be marketed at, who my audience would be, and where it could be promoted.
I was lucky that I was picked up straight away by a publisher, from what I've heard you should be prepared for a few rejection letters.
I don't have an agent, but again some of the bigger publishing houses ONLY deal with agents.
Obviously if you get traditionally published your publisher will sort out the ISBN, if you go down the self publishing route you will probably need an ISBN. Amazon make it very easy to self publish electronically, lots of other self publishing deals through people like Authorhouse can be found - you usually pay up front for the work and get so many copies, and then once those copies are sold out they are printed on demand.
If you do self-publish it is very unlikely that you will see your book on the shelves of the major book stores such as Waterstones, Blackwells etc....as they tend not to stock them unless you are a "local author".
Sorry - lots of rambling, hopefully some of it useful.
Best of luck with the writing and getting it published.
