Balance sheet is an accumulation over time. Income statement is a point in time …
So if you want to know how company X performed in year Y - you would look at the income statement for year Y - and see if the company made a profit or a loss. If it made a profit - it did well that year , else it did badly.
But if you want to compare how company X did vis-a-vis company A - also in the same business - you need to look at the progressive performance for multiple years. then you need to look at the balance sheets of the two companies.
Also remember the age old statements
A = L + E
E = Equity.
A component of Equity = Retained Earnings
And Retained Earnings = Net Income (from Income statement) - Dividends paid.
So the income statement (periodic statement) feeds into the Balance sheet (cumulative statement).
I think the statements becomes somewhat clearer if you take a look at some of the sample Balance Sheets and Income Statements in the book. The Balance Sheet typically provides you information at the year end and that’s it (although I vaguely remember seeing items such a beginning value and year end value but let’s ignore that for now). The Income statement on the other hand starts with your revenue and then lists all the expenses you incurred to end up at Net Income (See Exhibit 1 in Reading 25). So in a way it shows you what went on over the course of the year. At least that is my understanding of it.