Book value less than zero?

Yup. Rather counterintuitive.
Jim explained the rationale, although I can't find that post just now.

One way is "When a company buys back shares, it usually does so at prices far in excess of book value. So in essence, the company is retiring more than 100% of book value per share with each repurchase. This can cause book value to go negative."

Therefore it could mean that the company is operating with no assets (on the books). If they are making a profit that means that they are making on profit on NO assets. Yowsa!!!

Of course, if they are not making a profit you don't want to touch them.

So you look at earnings, 1 & 5 year revenue growth %, and if those are doing good it's probably a good company, but just the book value on paper is negative.

There are 31 companies in the S&P500 with negative BV. The first several have good 5 year revenue growth. Then we have BA with 5yr rev grth at -13%.

These have double digit 5yr rev grth %:
AZO
BKNG
DPZ
TDG
FICO
SBAC
YUM
LOW
VRSN
HLT