I think "The man who I thought was thoroughly honest proved to be a swindler" is acceptable. But my book says the answer should be "whom"
98815
The use of nominative "whom" (as in “persons whom it is foreseeable are likely to...”)
156993
"That is the lady (which / that / who / whom) I told you about." Which is correct?
98007
Can I use "that" instead of "who"?
76427
Is "It is you who are mistaken!" correct?
59180
When to use what or who
131090
On the singular vs plural aspects of "who"
27383
User: She, He, She or He, or They?
113847
What is the person called whom you give a recommendation?
80070
SWR surname that in fact describes or applies to the human in question?
123040
Mistaking versus mistaken
25183
How to use "who" vs. "that"
84256
Should I use "who" or "which" when referring to a specific animal species?
105272
Indian English, use of who
81637
Honest and Deceitful Professors Problem
90029
Is it ok to answer based someone's older answer on SE?
112142
Whoever or whomever: 'happy for ___ has the pleasure of working with you next.'
57982
Unnecessary pronouns: "The President he issued..."