The first thing in my Facebook feed this morning.

FREE TEXTBOOKS:

Chemistry/Physics/Math simulations: (useful for visualising how equations explain real world phenomena) [phet.colorado.edu]

[openstax.org]

Astronomy:
[teachastronomy.com]

Biology:
[learner.org]
[www2.estrellamountain.edu]
[biologycorner.com] (Also has Anatomy and Physiology/Evolution/Genetics)

Chemistry:
[chem1.com]
[acs.org]
[preparatorychemistry.com]
[www2.chemistry.msu.edu]
Interactive Periodic Table: [ptable.com]

Microbiology:
[microbiologybook.org]
[textbookofbacteriology.net]

Physics:
[physicsdatabase.com]
[physics.info]
[lightandmatter.com]
[flippingphysics.com]
[physicsclassroom.com]

[sciencebooksonline.info]
[open.umn.edu]
[nature.com]

National Academy Press has free full reports on everything from Climate Change, Evolution, Meteorology, Biotechnology - you name it, they have it: [nap.edu]

MORE FREE TEXTBOOKS & E-BOOKS
Bookboon: [bookboon.com]
Books Should Be Free: [loyalbooks.com]
College Open Textbooks: [collegeopentextbooks.org]
E-books Directory: [e-booksdirectory.com]
FullBooks: [fullbooks.com]
LibriVox: [librivox.org]
Mark Bishop’s Chemistry: [preparatorychemistry.com]
Open Textbook Library: [open.umn.edu]
Open Textbooks BCcampus: [open.bccampus.ca]
Open Textbooks: [oerconsortium.org]
OpenStax CNX: [cnx.org]
Textbook Revolution: [textbookrevolution.org]
WikiBooks: [en.wikibooks.org]