in this case, a book for memories and not one for bank notes or money.

Contrary to what one may conclude upon reading "engagement", in this day and age such term was meant towards a private understanding between two people. Not necessarily pertaining to marriage. As defined by the Oxford English Dictionary, an engagement in this case would be "A formal promise, agreement, undertaking, covenant."

http://www.oed.com.jpllnet.sfsu.edu/view/Entry/62197?redirectedFrom=engagement#eid

The narrator often switches between the minds of Elinor, and that of Marianne's. The usage of "mama" gives readers a clear knowledge of whose mind we are in - in this case, we are in the younger character (Marianne's) mind. Elinor, the older character, typically refers to her mother as Mrs. Dashwood; much more maturely. This shows the way that ladies carried themselves in this time.