I have just started learning German on my own through online material. On one website I found an example which used -es with the adjective 'neu':
Ein NEUES Auto
From what I have understood, the reasoning is the adjective needs to agree with the gender, case, and the article appearing before the noun. In the sentence "Das neue Buch ist da" the word Buch (book) is neuter hence the article 'das', and nominative. So why doesn't 'neu' (new) take -es ending becoming neues? Is there any rule for when -es applies to adjectives and when not?
Would be helpful if I am given any good and UPDATED grammar book recommendations!