Aziraphale gave away one book, not two, for the meeting/Ball
I’ve seen a lot of people post about the multiple books Aziraphale gave away and how surprising it was. It is definitely notable that he gave any books away, but I think the number and context are important.
He went to Mr. Arnold first because he would play a key role in the Ball. After the music shop owner directly said that nothing could get him to attend the meeting, Aziraphale offered him the Doctor Who annual. However, it was in exchange for bringing a harpsichord and a quartet of musicians, with less than 12 hours notice, to the meeting. So it wasn’t just bribery to attend- it was payment for a performing musician. I have no idea what something like the annual would be worth, but I assume it was fair compensation.
The second book, the magic book for Mr. Mott, he offered to lend, not give it.
So Crowley was definitely surprised that Aziraphale gave away a book, but it was just the one and it had a plausible explanation at the time.
