Thankfully this one is built of many redundant layers instead of just one layer of metal.

Very clear.

Secondary examples include:

Dictionaries, Encyclopedias (also considered tertiary);

Tertiary examples include:

Dictionaries and Encyclopedias (also considered secondary);

They do have a handy table later though:

| Primary | diaries - world war |

| Secondary | biography - world war |

| Tertiary | encyclopedia - world war |