The "Odd Thomas" series and the several "Ender's Game" sequels are available commercially; they aren't free, though.
Jack London's "The Call of the Wild" very much a compliment to "White Fang". It is public domain and available from a number of sites including:
http://manybooks.net/titles/londonjaetext95callw10.html Its a great book, but hardly the fun/humorous book you seem to be looking for.
You might browse around at ManyBooks.net. Humor is a rough genre for recommendations. Personal taste varies massively when it comes to what is funny or comic. One author whose works are mostly in the public domain that I like it P. G. Wodehouse (
http://manybooks.net/authors/wodehous.html).
Another is Mark Twain. This collection of short and very short stories of his might be a good start:
http://manybooks.net/titles/twainmaretext94beqst12.html