Best Practices for Graphics and Icons in Book Design

This is a really broad question and the complete answer is "to learn all about graphic formats, printing processes and how InDesign bridges them." 🙂

But a few guidelines —

• Keep your graphics as separate files. NEVER cut and paste elements from one app or display into InDesign. It works, for quick work, but pasted graphics are sort of incomplete and will cause all kinds of headaches if you're doing anything much more demanding than a one-shot flyer.
• Use the right graphics format for each image type. In general, this means —
  • JPEG for raster/photo/pixel-based images. PNG is an acceptable substitute but was not really designed for print and while some designers swear by it and have no problems, it can also get messy in certain kinds of color export. The very best option for raster images is Photoshop's PSD format, which carries extra information ID can use to optimize things like resolution and color export.
  • If you need transparency (as for a background around an icon), you have to use PNG or PSD. You can also use TIFF but that's an increasingly obsolete format and can be very bulky in size.
  • Use PDF for vector images. The only better format is Illustrator's AI format, whch like PSD is a fully editable format that meshes very well with ID's needs.
  • Both PSD and AI assume you have some facility with Photoshop and Illustrator; stay with the more generic formats until you do.
  • Don't use GIF or SVG for print. (Both are web-based formats best left to web pages and sometimes e-books. The only advantages of GIF are transparency and animation; PSD/PNG/AI are better if you need transparency, and animation is moot outside a web-based platform.)
• Use Place to import and place your graphics. You can drag and drop, but it's better to use the formal process of Place until you have experience with how images get into InDesign.
• Make sure raster images (JPEG, PSD and if you insist, PNG) have enough resolution for your final print export. This usually means at least 300ppi as they are placed in the document, which you can verify by selecting the image and looking at the Info panel. If the Effective PPI value isn't at least 300, you may want to find a higher resolution image file or reduce the placed size of the image. (Variations don't matter much, as export to PDF will convert all images to a selected value, usually that 300ppi. So, more is better, within reason, but it doesn't matter if you have images that are 300, 320, 382, etc. Effective PPI.)
• Generally, use RGB images, even for print. The need to use CMYK images, which used to be a requirement, is largely obsolete and for specialized projects.

All of this assumes you're printing using standard offset or process modes at a nominal 300 dpi. (DPI is the measurement of resolution on paper; PPI is the measurement of images in digital form. They are not the same thing.)

Your best practice overall is to pick a format and a printer and use their specifications for everything  —size, layout, margins, bleed, PDF standard, and export PPI. You will almost never print directly from ID, but export to PDF, even for in-house use. Everything just works better through a PDF.

And this all assumes print in the first place. If you plan to export to PDF only, there are some slighht variations. If you plan to export to e-book (EPUB), there are extra steps to format and pre-process images as well.

That's the basics. Lots and lots under those simple summaries. 🙂