Well, yes. Marilyn gave you the right advice. It is better for you to read Anderson books and booklets first and only after them, the fiction works about the Ingalls and the Wilders.
I must confess that I've read all of these or almost all. I loved both, fictional and historic works. Generally I would order five or six Ingalls books every three months (when the change favorized me, now I can't), so my collection is pretty complete.
I enjoyed Martha's, Charlotte's and Caroline's Years (I don't have the complete collection of the last two of them, but well...
) for different reasons that I likes Lauras books. These are not only interesting for it mentions Laura's female relatives, but also for you can learn how people lived in XVIII Century in Scotland, at the very beginning of XIX Century near Boston (right after American Independence) and in the 1840's when Westward movement just started. I love these books for even if I knwo they are not accurate about Ingalls family they are a good material for an historian like me, who is focused in cultural and social history.
My own recommandation is that you read these before any other book related to the Ingallses (all of them by William T. Anderson)
-"The Story of the Ingalls" (Before and after the Little House Books)
-"The Story of the Wilders" (Almanzo and family)
-"Laura Wilder of Mansfield" (The Ozark years)
-"Laura's Rose" (Rose Wilder Lane's story)
-"Wilder in the West" (Eliza Jane's story)
-"The Horn Book's Laura Ingalls Wilder (Reprint of 1943 and 1953 Wilder issues)
-"Laura Ingalls Wilder: the Iowa Story" (Account of the Ingalls' Years at Burr Oak)
-"The Walnut Grove Story of Laura Ingalls Wilder"
-"A Little House Sampler" Stories, essays and articles by Laura Ingalls and Rose Wilder Lane, settings by Anderson)
-"A Little House Reader" (same than above. I didn't read it, tough)
-"Laura Ingalls Wilder Country" by Anderson and kelly (Large 117 pages of color and black and white pictorial with background commentary)
-"Laura Ingalls Wilder- A biography (240 pages)
And also, if you like:
-"Laura" (Life f Laura Ingalls Wilder, by Donald Zochert) EXCELLENT
-"Westville Florida Years of Laura Ingalls Wilder", by Alene Warnock
-"Spring Valley-The LIW connection" (The year after the "First Four Years), by Mary Jo Dathe.
There are some other good titles about the food they ate and the music they sang. If you want them, I can give you the info.
And then, yes...Read all the other "Little House" years. I loved them all, for sure, and I'm sure you'll love them too.