While everyone writing a book puts in their best efforts to get an impeccable book out, an insider's perspective and assistance is needed to improve your chances of landing a contract.
Get an honest feedback on the merit, the manuscript has in its current form in structuring, writing, research, copy and proof to get help in improving it to fit the publishing standards.
You can get references to the kind of editors you could seek help from (if need be) before sending the manuscript to publishers.
Drop the first 5 chapters, a synopsis and index here as an attachment and will get back to you with a detailed feedback after reading the same.