Learn Icelandic: Ég les góða bók um sögulegt tímabil. - I read a good book about a historical period.

The verb lesa (to read) needs its object to be in the accusative case. Because bók is a feminine word, the adjective góð changes to its feminine accusative form, which is góða.

Since we are learning about time expressions, does um have a time meaning in this sentence?

Not here! While you will see um used for time expressions like um helgina (over the weekend), in this sentence it simply means 'about' (a book about something). Whether it means 'about' or refers to time, um always takes the accusative case.

Why does the adjective sögulegt end in -t?

It describes the noun tímabil, which is a neuter word. The base dictionary form of the adjective is sögulegur, but it changes to sögulegt to match a neuter noun in the accusative case.

How is the word tímabil built?

It is a compound word made of tími, which means 'time', and bil, which means 'space' or 'interval'. Together, they literally mean a 'time interval', which we translate as 'period'.

Is les the present or past tense here?