SOLVED. Adult Science Fiction, Detective novel. Man joins interplanetary investigation service. Character named Tog with Chinese surname. Read in 1970's.

[Marc *Dark Reader with a Thousand Young! Iä!*]

Here are some SFF double-sided book lists of the type you describe:

https://en.wikipedia.org/wiki/List_of...

https://www.goodreads.com/list/show/1...

[Marc *Dark Reader with a Thousand Young! Iä!*]

I know those lists don't show what you remember as the flipside book, but maybe this list of editions from the Internet Speculative Fiction Database will help track it down:

https://www.isfdb.org/cgi-bin/title.c...

[Marc *Dark Reader with a Thousand Young! Iä!*]

Leslie wrote: "Planetary Agent X by Mack Reynolds 1965. A million thank yous"

Planetary Agent X (adding the link for the record). Interestingly it's paired with at least 2 books in the Goodreads database: Planetary Agent X / Behold the Stars and Rival Rigelians / Planetary Agent X.

[Kris]

Glad you found your book, Leslie.

Main novel: Planetary Agent X (book 1 of the United Planets series) by Mack Reynolds

I copied some book details to the topic header, as an example of the information we need at the top of the page.