Hi..
This is a secondary dominant..
B7 is V of E, which is V of A..
So a V of V..

You could also try a V of any of the other chords too..
Say V of ii, F#7 to Bm in A.. or V of iii, Ab7 to C#m..
V of vi is common too..So C#7 to F#m..

(All these examples are in A..)