The word 'editor' is one of the most unhelpfully elastic words out there. There are several different editors involved in the book production process - here's a quick taxonomy:
Book editor (for a multi-contributor book) - the person with overall editorial responsibility for commissioning and approving the various contributions, and typically will write/compile any prelims and endmatter too, eg preface, introduction, bibliography. This is typically the person who has the direct contractual relationship with the publisher, and who is responsible for the further dissemination of royalties if appropriate. This could be you if you're working with a collection of authors. (There are also series editors, who do a similar job at a higher level, identifying authors for new books in an existing series and ensuring that all new books meet the appropriate quality requirements and are consistent in tone and emphasis with the other books in the series.)
Commissioning editor (also known as acquisitions editor) - this is someone employed by the publisher to develop a list in a particular area (eg business books). They will have a clear strategy for how they intend to build that list, and if your proposal doesn't fit with that strategy they're unlikely to accept it, even if it has real merit. Commissioning editors work with the book editor or, more usually, the author (you!) on editorial issues such as the overall structure, level and length of the book, and manage contractual issues, deadlines and so on.
Development editor - a professional editor who gives more detailed formative feedback on the draft manuscript, assessing structure, consistency of tone, appropriateness for target market, etc, making recommendations on how to improve it plus identifying any systematic issues such as permissions requirements, libel etc. Not all publishers provide development editors - at Practical Inspiration we think this stage is hugely important. If your publisher doesn't provide this kind of support, it's worth investigating whether you can employ someone to work with you directly.
Copy-editor - another professional editor managed by the publisher - but often working as a freelance rather than as an employee - who checks the final manuscript line by line for typos/errors, picks up any repetition or ambiguity, queries inconsistencies or missing references, and creates the structural mark-up for the typesetter. They are focused on polishing the manuscript and correcting errors rather than suggesting significant structural changes or improvements.
So if someone's offering their services as an 'editor', make sure that you're clear on what exactly they mean by that before you agree to work together!