I understand it out of context as "in the direction of the subject of shops", especially as it's introduced with "turned", and the sentence mentions a restaurant, "shop" being an informal word for a business.
We had the restaurant mostly to ourselves, and talk turned shop-wise.
We had the restaurant mostly to ourselves, and talk turned to the subject of shops.
However, in the context of the full passage, it's clear they're talking about their work, in this case dictionary writing. The author has used "to talk shop" (a standard idiomatic phrase) in a non standard (but still understandable) way with the word "turned" and the suffix "-wise".
We had the restaurant mostly to ourselves, and talk turned to the subject of work.
See shop from Wikipedia (definitions 4 & 8):
shop (plural shops)
- An establishment that sells goods or services to the public; originally only a physical location, but now a virtual establishment as well.
- A place where things are manufactured or crafted; a workshop.
- A large garage where vehicle mechanics work.
- Workplace; office. Used mainly in expressions such as shop talk, closed shop and shop floor.
- A variety of classes taught in junior or senior high school that teach vocational skill.
- An establishment where a barber or beautician works.
a barber shop
- An act of shopping, especially routine shopping for food and other domestic supplies.
This is where I do my weekly shop.
- (figuratively, uncountable) Discussion of business or professional affairs.
See -wise from Wikipedia:
- in the direction or orientation of
The gaoler slowly turned the key clockwise.
- in the manner of
You need to follow the instructions carefully; otherwise, the project may not turn out.
Contrariwise, it could be a good idea.
- in the matter of; with regard to
This morning looks promising, weather-wise.
- One (thing) at a time
Add the reagent dropwise to the solution.
The usage is rare in this particular case and is done for humour or a light style.