Difference between 'viel' and 'viele'

What is the difference between viel and viele?

→ used with a noun in the singular form, for example:

Er isst viel Gemüse.

Ich habe nicht viel Zeit.

→ used with a noun in the plural form, for example:

Wir haben viele Bücher.

Im Zentrum waren viele Leute.

See words in context to better understand their meaning and use.