commas - If you're using a quote with a period but do not want to end the sentence, do you keep the period?

For example. If I want to quote a passage from a writing, that says

The cake is not a lie.

and my sentence is:

The book by author states that "The cake is not a lie." however studies show that the cake is a lie.

Is it OK to replace the period with a comma to make the sentence flow?

Relevant: Also what do you do with the capital "T" Are you allowed to lowercase it?

The book by author states "the cake is not a lie," however studies show that the cake is a lie.

I usually just work around this by switching up the structure of the sentence, but sometimes I really want to phrase something a particular way.

Answer

The best source for such questions is The Chicago Manual of Style. (By "best," I mean that it is most supportive of my existing prejudices.)

15th Edition, Section 11.8 permits these changes (and others) to quotations:

• The intial letter may be changed to a capital or lowercase letter


• The final period may be omitted or changed to a comma