Swallows & Amazons by Arthur Ransome is one of the books I discovered thanks to How the Heather Looks. This book was written in the 1930s for children (I'm guessing in the 6-14 age range), but this doesn't make it less enjoyable for adults.
If I read this as a child, my life would have been a bit different. Just the idea that I had the ability to "run away" and be in nature, because other kids did it too...my mother would have never heard the end of my scheming. I devoured this book in a matter of days and was only consoled in finishing the book by knowing this is just the first in a series.
I'm currently finishing-up the second book, Swallowdale, which takes place more on land than "at sea." It is a nice change of pace to see these siblings play explorer and climb a "mountain," though it is not as strong as the second book. I've read reviews stating that the fourth in the series may be thr strongest, but I'll let you know when I get there. As in the first book, Ransome continues to sprinkle his illustrations of the kid's adventures throughout the story. I have to say I really enjoy his illustrations, although not very detailed nor perfect.
As for the very first book, Swallows & Amazons: it is an absolutely amazing story that I look forward to reading to my friend's kids and rereading myself. I highly suggest you pick-up this book, especially for summer reading.