A substantive edit usually takes place after the first draft of a manuscript is complete. A substantive edit looks at all aspects of your work: from your writing style and authorial voice to the grammar and spelling. It picks-up on inconsistencies, inaccuracies, structural issues, potential pitfalls and loose ends. Your editor may query any aspect of your narrative or your assertions if it is felt this may create confusion for the reader. As the name suggests, this work can often be substantial and result in the author working with the editor to rewrite, improve and clarify certain sections of the book. Your work is likely to need a proofread after the substantive edit to ensure it is ready for publication.
This service works well for a manuscript that is at the final stages of the writing process and has often been through a second or third draft already. The copy-edit picks up on spelling, grammar and consistency issues. The editor may comment and feedback to the author on certain points, but this usually requires a brief clarification rather than a rewrite. A copy-edit results in a clean manuscript that is proofread and ready for next steps.
If you are confident that your book is coherent and concise – but suspect that there are a few errors in it – then a proofread may well be all you need. A proofread will correct spelling mistakes and grammatical errors, but does not comment on the content of the book (unless, of course, there are glaring errors or paragraphs missing). A proofread is highly recommended before either design or representation processes are undertaken.