TechWhirl (TECHWR-L) is a resource for technical writing and technical communications professionals of all experience levels and in all industries to share their experiences and acquire information.
For two decades, technical communicators have turned to TechWhirl to ask and answer questions about the always-changing world of technical communications, such as tools, skills, career paths, methodologies, and emerging industries. The TechWhirl Archives and magazine, created for, by and about technical writers, offer a wealth of knowledge to everyone with an interest in any aspect of technical communications.
Depending on the level of editing and the type of document, your question
could have many answers.
I like _Technical Editing_ by Judith A. Tarutz.
Remember, editing is not an exact science (any more than writing is), and
the circumstances and needs of the situation dictate what's appropriate. Are
you asking about books on "how to edit," or are you looking for style guides
(amply covered in the TECHWR-L archives)?