Sherlock stares at the book through the window. It's not really his area. Most suicides, with a few exceptions, are boring, uncomplicated, and simple enough for the police to figure out. And Sherlock's usually only called out after there's a body, so the prevention part is no use. There's really no reason to get the book.
But, unbidden, his mind strays to pools of water with drops of blood slowly diffusing into pink clouds. To the glint of light on a tiny bit of metal and the way it sent sparkles across the room. To utter, utter stillness broken by a screaming child. Sherlock wrenches these thoughts away and forcibly shoves them back into their tiny basement room in his mind palace and bolts the door. But there are other thoughts that nudge him for attention as well. A quiet army doctor with deep, tired lines on his face and demons in his dreams. A gun that sits in the bedside table. A haunted look that appears in warm blue eyes when things get too quiet or those demons get a little too close or the dreams a little too vivid.
Without hesitation, Sherlock pushes inside the store and buys the book. Just in case. Might be useful one day, he thinks as he tucks the book carefully into his bag.