I want to expand a df with time column.

Here I have 2 df:

```
book1 <- data.frame(
  id = c('A1', 'A2', 'A3')
)
book2 <- data.frame(
  year = c('2000', '2010', '2020')  
)

> book1
  id
1 A1
2 A2
3 A3

> book2
  year
1 2000
2 2010
3 2020

```

What I expect is

```
  id year
1 A1 2000
2 A1 2010
3 A1 2020
4 A2 2000
5 A2 2010
6 A2 2020
7 A3 2000
8 A3 2010
9 A3 2020

```

Could anyone help me to do this? Thank you!