Hi,
Well, reading the PMBoK Guide beforehand is undoubtedly not a failure; however, it makes more sense to use it as a companion throughout the BC to deepen your knowledge and understanding.
IMPORTANT: PMBoK Guide 6 is no longer valid and has already been officially retired by PMI.
The content of this edition in terms of Processes, ITTOs, and so on has been moved to the official "Processes Practice Guide" (can be downloaded as a member.
By the way, it will be interesting to see what PMI does with this Processes Guide, since the Processes will have a comeback in the upcoming new 8th Edition of the PMBOK Guide.
In terms of readings and recommended sources, you should not only refer to the PMBoK Guide but also to the Agile Practitioner Guide The BA Guide and a few others. Refer to offficial list of references by PMI at pmi.org.
BR