So, you have read, re-read and edited your crime novel. Maybe you have had it independently read and received a critique. You have to send it out there at some point and try to avoid holding onto it too long. As a mentor at my local university this is something I encounter a lot with my students.....being slow to let the assignment/project go. There is always the temptation to hang on and look again and change something else. It has to stop!
Study the Writers & Artists' Year Book - looking for an agent or publisher that specialises in your type of book. It is good to have a think about the sub-genre your book belongs to. Is it a "vintage mystery," (this is, I believe, where my Edith Horton novels fit!) Maybe your book is a "grip lit" novel. These are really popular at the moment and would possibly be of interest to one of the more mainstream publishers. Other sub-genres include: "comedy or light-hearted crime," or the more mainstream crime novel.
You will have your own favourite novels/writers. Look at their websites/publishers and use these to draw up your own list.