ISB ~ Acts 1 v1

the first book:The Gospel of Luke, which has a similar but more expanded dedication (Lk 1:1-4). This link with the preface of the Gospel has a direct bearing on the composition of Acts, i.e., Luke has gone to the same painstaking lengths to separate fact from fiction in preserving the historical memory of the earliest Christians. See note on Lk 1:2.

began to do and teach:Implies that what Jesus began to do in Luke he continues to do through his disciples in Acts. The Spirit directs this mission of the Church and is the driving force behind it (1:8; Lk 24:46-49).