Brad is a homeless guy and his only tenable option is to sell the spellbook.
Is it? Brad doesn't have much imagination. Maybe he shouldn't be a wizard.
Brad has just made the scientific discovery of the century: actual working magic! And he can demonstrate it! Forget selling the book, use it to demonstrate real magic to the world (and make a lot of money doing so).
Win All The Skeptic Prizes
First, Brad should make a quick buck off the various prizes offered by skeptic groups around the world for anyone who can demonstrate, under scientific conditions, paranormal activity. The most famous is James Randi's One Million Dollar Paranormal Challenge, but that ended in 2015. There's plenty more to be had. Brad should contact them and clean house. The prizes are anywhere from a few thousand dollars, to $100,000 each from Australian Skeptics and Independent Investigations Group.
This will not only solve Brad's financial problems, but it will bootstrap his credibility. Not a single one of these prizes has ever been claimed, and he can sweep most of them up.
It also addresses the problem of getting killed, realistic or not. These groups are used to dealing with crackpots, so he can openly present himself to them with no fear. Once he's been publicly tested, the publicity will prevent any overt again against him... by someone... for some reason... Look, I'm not a big fan of "the secret shadow government will suppress magic" meme.
At this point he has some not entirely mutually exclusive choices for making money.
Auction The Book For Billions
This is probably the laziest way to make money.
After he's demonstrated its power via skeptical groups he could sell it via any number of auction houses. He'd give the book to the auction house on consignment, so they'd be responsible for protecting it, and there'd be no point in going after Brad. The auction house itself would also check its authenticity and stake their reputation on it.
What would somebody pay for real, working magic? Imagine what the space industry could do with anti-gravity. Imagine what the military could do with soldiers casting fireballs and lightning bolts and tossing tanks around with their minds. Imagine what natural disasters could be stopped with control of the elements: dissipate hurricanes, end droughts, dampen earthquakes, control flooding.
It would be worth BILLIONS.
Become An Entertainer
This is the second least ambitious thing to do: become the world's greatest stage magician. If there is a shadow government wanting to stamp out magic, this is also great cover: a real magician posing as a fake one is no threat to the fabric of society.
Be Tested By The Scientific Community
If Brad is feeling noble, he could share his discovery with the world!
Scientists will be clamoring to test something that defies everything we know about reality, and he can probably squeeze quite a bit of money out of people wanting to examine the book, witness his powers under scientific conditions, and try to replicate them. Control of gravity alone could revolutionize the world!
This would also protect Brad from being killed or the book stolen, once the secret of magic is out there's no reason to go after Brad.
Start A Business Selling Magic Books?
I mean, it could be copied, right? The problem here is once there's another copy, someone will copy it, so probably not a viable business.
Become A Super-Hero/Villain!
You command the elements! Go steal some things and/or rescue them!
This is the riskiest of them all. As a villain or vigilante with so much power he'd be hunted by the authorities. Like any good super-person he'd need a secret identity. It also means he can't go the skeptic prize route, that would link him to magic.