The two books that live in my mind as 'the first' in my young life were two very big books--one red and one blue covered--the first called 'Stories That Never Grow Old' and the second although I can't remember the title was a book of Fables. My parents always bought us books with moral lessons written in an engaging way.
When I started to read on my own, which was at quite a young age--maybe 5 years old--I began with the Louisa May Alcott series and another of same timeline but I can't remember it's name. I think the heroine was 'Rose' or 'Rosemarie'. The characters were similar to the Louisa May Alcott characters but the writing was much more flowery which is why I think I preferred the Alcott series.
I read well at 5 years old. In my home town then of Northampton, MA a child had to be six years old to start first grade and not before and my 6th birthday wouldn't be until Nov 30! Oh, my heart was broken. So, my father made an appointment with the principal of the local elementary school, took me with him and started to plead my case. Finally, the principal (with a bit of a humoring and condescending air) said oh then, you may choose any book from these bookshelves above my head and if you can read it I'll let you enter this September. I selected the biggest, fattest book I could reach and began to read it--smoothly and easily I might add. The principal gave in and I started school three months before the 'legal' time. It was a story repeated over and over in our family. Actually, my father had taught me to read phonetically and I'm sure I wouldn't have been able to tell the principal exactly what every line meant...but I must have done enough for his satisfaction.
In our home we always read poetry, both what my father wrote and that of others like Longfellow, Edgar Guest who were two of my favorites. But my first 'grown up' book is one I still read again whenever I miss the characters. My mother let me read her copy--"Jubilee Trail--about a wagon train trip from Boston to Santa Fe. It was also a light romance but I've always loved books where people on the fronteir and the descriptions included how they lived, what they wore, and , of course, what and how they ate. I still have a copy of that book!