There are many books about filing patents, however, this one: A Guide for Filing A Design Patent Application given for free by the United States Patent And Trademark Office is an authoritative and simple start.
The booklet starts giving the accepted definitions of the different kinds of patents. Since the matter of the booklet is "design patents", then the publication proceeds with good examples of what constitutes a "patentable design".
The Office examiners must comply with very strict regulations concerning the types of drawings that the applicant must use for the different scenarios or views that apply to the submitted design. There are many types of lines, and each one is for a very specific use.
For example, for this design of an Aquarium where the sides are transparent, the Office observes: "Note that elements visible behind transparent surfaces should be shown in light, full lines, not broken lines".
On the other hand, the broken lines are used for other purposes, like in the example they give of the paper cutter.
Surfaces must be shaded according the materials used: wood, cloth, cork, etc.
"In general terms, a 'utility patent' protects the way an article is used and works (35 U.S.C. 101), while a 'design patent' protects the way an article looks (35 U.S.C. 171). Both design and utility patents may be obtained on an article if invention resides both in its utility and ornamental appearance. While utility and design patents afford legally separate protection, the utility and ornamentality of an article are not easily separable. Articles of manufacture may possess both functional and ornamental characteristics.
The actual 'examination' entails checking for compliance with formalities, ensuring completeness of the drawing disclosure and a comparison of the claimed subject matter with the 'prior art.' 'Prior art' consists of issued patents and published materials. If the claimed subject matter is found to be patentable, the application will be 'allowed,' and instructions will be provided to applicant for completing the process to permit issuance as a patent."
Click here to download A Guide for Filing A Design Patent Application.