- bookboon // for accounting, business, economics & finance, engineering, IT & programming, languages, marketing & law, natural sciences, statistics & mathematics (+ career & study advice, strategy & management)
- booksee // for arts & photography, biographies & memoirs, business & investing, computers & internet, cooking, entertainment, health, history, home, law, literature & fiction, medicine, references, religion, science, sports, travel, and other categories
- boundless // for accounting, algebra, art history, biology, business, calculus, chemistry, communications, computer science, economics, education, finance, management, marketing, microbiology, music, physics, physiology, political science, psychology, sociology, statistics, U.S. history, world history, writing
- california learning resource network // for mathematics, science, history
- ck-12 // for elementary math, arithmetic, measurement, algebra, geometry, probability, statistics, trigonometry, analysis, calculus, earth science, life science, physical science, biology, chemistry, physics, sat exam prep, engineering, technology, astronomy, english, history
- college open textbook // for anthropology & archeology, art, biology & genetics, business, chemistry, computer science, economics, engineering & electronics, english & composition, health & nursing, history, languages & communication, law, literature, math, music, philosophy, physics, political science, psychology, science, sociology, statistics & probability
- ebooklobby // for arts & photography, biographies & memoirs, business, computers & internet, cooking, entertainment, health, home & garden, law, literature & fiction, sports, travel
- freemathbooks // for algebra, geometry, trigonometry, calculus, applied math, probability, analysis, statistics, and other sub-categories of mathematics
- global text project // for business, computing, education, health, science, social sciences
- openstax cnx // for arts, business, humanities, mathematics & statistics, science & technology, social sciences
- open culture // for art history, biology, business & management, chemistry, classics, computer science & information systems, earth science, economics & finance, education, engineering, history, linguistics, law, mathematics, music, philosophy, physics, political science, psychology, sociology
- open textbook library // for accounting and finance, business, management & marketing, computer science & information systems, economics, general education, humanities & language, law, mathematics & statistics, natural & physical sciences, social sciences
- textbook revolution // for biology, business & management, chemistry, computer science & technology, earth sciences, economics, engineering, environment, ESL, health sciences & medical, mathematics, physics, society and social sciences, sociology, world history
+ bonus
- bookstacks // popular classics
**If you know other helpful sites, please send me a message so I can add it to the list!