How do I use the API to download just the latest value for a time-series?

Use the limit=n parameter to get only the first n rows of a time-series. Here is an example of getting just the latest value of AAPL's stock price from the premium EOD data feed:

https://www.quandl.com/api/v3/datasets/EOD/AAPL.csv?limit=1&api_key=YOURAPIKEY