What is a word for when you condense a topic into a work that is all that's needed to understand it?

Solution 1:

Consider quintessential

the essence of a thing in its purest and most concentrated form

Solution 2:

you can say:

compendious (adj) compendium (n)

containing the substance of a subject, often an exclusive subject, in a brief form; concise: a compendious history of the world.

succint (adj)

marked by brevity and clarity; concise

precis (n, v)

a summary of the essentials of a text; abstract

abridge (v)

to shorten by omissions while retaining the basic contents: to abridge a reference book.

epitome (n)

Solution 3:

Depending on the context you could use any of:

• pithy (a pithy explanation of the EU)
• summary (in summary, the EU is ...)
• executive summary/abstract
• boils down to (The EU boils down to ...)
• in a nutshell (in a nutshell, the EU is ...)