The Mushroom Planet Books

I just finished (well, not just... like, yesterday...) reading the first book in the Mushroom Planet series, it's called The Wonderful Flight to the Mushroom Planet. It's a kids' series, it was one of the very first sci-fi books written for children, in the from 1954 onward. The author is Eleanor Cameron, this is the cover of the first book:

I read the whole series for the first time when I was in 6th grade, and I've probably read it three or four times. I JUST found the second book on Amazon, along with a few of the other books. Luckily, the second book I found was a hardcover in good used condition for 12.99. Some sellers wanted upwards of 50.00 for them! And one even wanted freaking 600.00!!!! WTF?!? What book is worth that much money??? Anyways. It's a really fun series, and I'm enjoying reading it again from a more mature perspective :)