Highly rated platforms for ebooks
Novice Level:
Project Gutenberg
Offers a variety of free ebooks on basic economics and personal finance.
OpenStax:
Provides textbooks on introductory financial topics, suitable for beginners.

Intermediate Level:
Google Books
The "Free Ebooks" section often contains intermediate-level finance books.
JSTOR:
Provides free access to various finance journals and books, some of which are at an intermediate level.

Expert Level:
SSRN:
The Social Science Research Network offers free academic papers and ebooks on specialized finance topics.
MIT OpenCourseWare:
Provides free course materials, including ebooks, for advanced finance courses.

You can search for these platforms and navigate to their finance or economics section to find ebooks that suit your level of expertise.