Bookboon // For Accounting, Business, Economics & Finance, Engineering, IT & Programming, Languages,

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 let me know so I can add it to the list!