Learn German: Ich habe neunzig Bücher.

Just like in English, you take the base number neun (nine) and add a suffix. In German, this suffix is -zig, which works exactly like the English '-ty'.

How do you pronounce the -ig at the end of neunzig?

In standard German, the -ig at the end of a word is pronounced like a soft 'ich' sound, not a hard 'g'. It sounds similar to the 'h' in the English word 'huge'.

What is the singular form of Bücher (books)?

The singular is Buch. To make this word plural, German adds an -er to the end and changes the vowel u to the umlaut ü, creating Bücher.

Do numbers like neunzig change their ending depending on the noun they describe?

No. Unlike descriptive adjectives, numbers from two upwards generally do not change their endings. You always use neunzig, whether you are counting books, cars, or people.