The current documentation for SOQL Date Literals does not have N_X_AGO on its list. In fact I had never seen the AGO date literals on any version of the documentation. So I am wondering if there are more "hidden" date literals and which rock they are hiding under.
These are not the date literals you're looking for....... :-)
The trick is that the filters that are available for filter criteria in list views and reports are secretly available in SOQL as well. You'll find them by checking out this link below. Most of these are already on your list, but the missing ones are here as well, just replace spaces with underscores!
The link provided in the accepted answer does not include the hidden date literals, so I am not sure if SFDC modified their documentation page since then.
AFAIK, these are not specifically documented, but below is a list that I think is complete.
Here are the places that I sourced the list:
YESTERDAY TODAY TOMORROW LAST_WEEK THIS_WEEK NEXT_WEEK LAST_MONTH THIS_MONTH NEXT_MONTH LAST_90_DAYS NEXT_90_DAYS THIS_QUARTER LAST_QUARTER NEXT_QUARTER THIS_YEAR LAST_YEAR NEXT_YEAR THIS_FISCAL_QUARTER LAST_FISCAL_QUARTER NEXT_FISCAL_QUARTER THIS_FISCAL_YEAR LAST_FISCAL_YEAR NEXT_FISCAL_YEAR NEXT_N_DAYS LAST_N_DAYS N_DAYS_AGO NEXT_N_WEEKS LAST_N_WEEKS N_WEEKS_AGO NEXT_N_MONTHS LAST_N_MONTHS N_MONTHS_AGO NEXT_N_QUARTERS LAST_N_QUARTERS N_QUARTERS_AGO NEXT_N_YEARS LAST_N_YEARS N_YEARS_AGO NEXT_N_FISCAL_QUARTERS LAST_N_FISCAL_QUARTERS N_FISCAL_QUARTERS_AGO NEXT_N_FISCAL_YEARS LAST_N_FISCAL_YEARS N_FISCAL_YEARS_AGO