Fred and George Weasley: And because their sneaky achievements deserve to be memorialized forever (duh), Pottermore created an awesome infographic of a history of their mischief.
Timeline Of Events: 1881 Through 1956 Harry Potter Infographic
DIY: Tea Stained Hogwarts Letter - printable and tutorial
Harry going on a no-sugar diet is literally the most ridiculous sentence I've ever read. Harry Potter and the cursed child shouldn't have Harry Potter in the title.
Get ideas for a Harry Potter themed playroom by incorporating Harry's magical world into your Muggle one.
Not strictly speaking a book post, but I loved these Harry Potter props that a friend sent me in work earlier. I love the marginalia in the potions book from The Half Blood Prince.
Rowling's Plot Spreadsheet This one is from Order of the Phoenix. A spreadsheet plot written out by J. Her approach to spreadsheet plotting is to divide the columns by chapter number, story timeline, chapter title, main plots and subplots.