。
Let's walk to the bookstore.
Literal
Walk-and bookstore [to-へ] let's-go (polite).
歩いて as the te-form chains the manner of motion ('walking') to the main verb 行きましょう. へ marks the direction. The whole pattern [manner verb-te]+[motion verb] is the standard way to say 'go by X-ing': 歩いて行く (go on foot), 走って帰る (run home), 泳いで渡る (swim across).