Do these books need to be read in sequence or do they stand alone?  I was given book # 27 and have #11.  I don't know if I should go back and start with #1. 

although each story deals with a family member of the winslow family (and there are a LOT of them), they can be read as standalones

I would definitely recommend reading number 1 at least. It introduces the House of Winslow. The earlier books talk about Gilbert Winslow's diary or book and he's introduced in the first book.

Nancy

I have the whole series, and I read them in order.  I fully agree with reading the first one and  after looking at the list of titles just now, I think 2 and 3 might be a good idea, too.  These first ones are set during Gilbert's lifetime and that of his decendants who knew him personally.  They set the tone for the family's identity and legacy, and after that, it's assumed the reader knows this.  For example, Gilbert's journal pops up over and over, and it's assumed the reader knows about it, but virtually all content about him and the journal is contained in those first two or three books.  But after that, I wouldn't hesitate to read any of them in any order, whichever settings interest you the most, or that you have easy access to.