By “Classical Reformed” I mean the Reformers, Puritans, Particular Baptists, Dutch Reformed scholastics, and later Reformed writers—Calvin, Owen, Bunyan, Watson, Flavel, Edwards, Gill, Spurgeon, Warfield, Hodge, etc.—there is a remarkable amount available legally for free.
For your interests, I’d start with these:
- Monergism — Best overall starting point. It is explicitly committed to historic Reformed theology and has a huge topical library plus 1,500+ free Reformed and Puritan ebooks, including Calvin, Owen, Watson, Edwards, Warfield, Berkhof, Spurgeon, and many others. Many works are available in PDF/ePub or readable online. Monergism
- Chapel Library — Especially recommended for you. Chapel Library’s confessional standard is the 1689 Second London Baptist Confession, and it distributes Christ-centered literature without charge. Its catalog approaches 1,000 titles and includes Calvin, Spurgeon, Bunyan, Pink, Ryle, Bonar, and numerous Puritans. Chapel Library
- Post-Reformation Digital Library (PRDL) — Best for serious historical research. This is an extraordinary academic index of primary-source material from the Reformation and post-Reformation period. It currently catalogs more than 110,000 titles and 149,000 volumes, including tens of thousands associated with the Reformed tradition. Particularly useful when researching obscure Puritan and Reformed scholastic sources. Post-Reformation Digital Library
- Christian Classics Ethereal Library (CCEL) — Best broad classics library. Not exclusively Reformed, so theological discernment is required, but it contains many classic works from Calvin, Edwards, Hodge, Schaff, Augustine, and others. Reformed Theological Seminary includes CCEL among its recommended free research databases. Reformed Theological Seminary
- Puritan Reformed Theological Seminary Open Access Resources — Best research portal. This is particularly useful because PRTS has already assembled links to numerous serious open-access theological collections, including PRDL, Puritan resources, open-access theological libraries, Reformation resources, Spurgeon archives, and specialized Reformed collections. WPLibrary
- Internet Archive — Best for scans of old editions. Search by author/title for original scans of Puritan, Presbyterian, Particular Baptist, and Reformed works. This becomes especially useful when a work isn’t transcribed elsewhere.
- Google Books — Excellent for public-domain theological books. Particularly useful for 18th- and 19th-century editions of commentaries, systematic theologies, sermon collections, and historical works. RTS specifically includes Google Books among its research resources. Reformed Theological Seminary
- Project Gutenberg — Good for clean downloadable editions. Its Reformed collection isn’t nearly as focused as Monergism, but public-domain editions are often available in excellent EPUB/Kindle-friendly formats. RTS also lists Gutenberg among its research databases. Reformed Theological Seminary
- Westminster Theological Seminary Resources — Reformed academic material. Useful for more scholarly Reformed material, including lectures, archived media, theological writing resources, Westminster-related research, and access points to Reformed scholarship. Westminster Theological Seminary
- Reformed Theological Seminary Library Resources — Excellent gateway page. Rather than being primarily a repository itself, RTS maintains a useful directory pointing to CCEL, Project Gutenberg, Monergism, open-access theological databases, historic Bible collections, dissertations, journals, and other research resources. Reformed Theological Seminary
For a Reformed Baptist library
Given your particular interest in confessional Reformed Baptist theology, I’d bookmark them roughly in this order:
Chapel Library → Monergism → PRDL → CCEL → Internet Archive → PRTS Open Access Resources.
There is a useful distinction between them. Chapel Library and Monergism are curated theological libraries; PRDL is a scholarly primary-source research tool; CCEL, Internet Archive, Google Books, and Gutenberg are broad digital libraries where you need to know what authors you’re looking for.
For your Romans commentary project, PRDL deserves special attention. It isn’t merely a collection of popular Puritan books: it catalogs primary sources involving the Reformed churches, Puritans, Westminster Assembly, Synod of Dort, historic commentaries, theological disputations, confessions, catechisms, and early-modern Reformed scholarship. Post-Reformation Digital Library