Useful sites to check readability:
Lexical Tutor: https://www.lextutor.ca/
Oxford Text Checker: https://www.oxfordlearnersdictionaries.com/textchecker/
Text Inspector: http://www.englishprofile.org/wordlists/text-inspector
VocabKitchen: https://www.vocabkitchen.com/profile
Pedagogical grammars – some recommended grammars for teachers and/or learners:
Carter, R. and McCarthy, M. (2006) Cambridge Grammar of English:
A Comprehensive Guide. Cambridge University Press.
Murphy, R. (2019) English Grammar in Use (5th Edition). Cambridge University Press.
Parrott, M. (2010) Grammar for English Language Teachers (2nd Edition). Cambridge University Press.
Swan, M. (2016) Practical English Usage (4th Edition). Oxford University Press.
Thornbury, S. (2019) 101 Grammar Questions. Cambridge University Press.
Thornbury, S. (2017) About Language: Tasks for teachers of English (2nd Edition). Cambridge University Press.
Online corpus sites:
English corpora: https://www.english-corpora.org/ (includes the Corpus of Contemporary American English [COCA], as well as many other specialized corpora)
Sketch Engine for Language Learning (SKELL):
https://www.sketchengine.eu/skell/ – provides examples, collocations and a thesaurus for learners of English
Webcorp: https://www.webcorp.org.uk/live/ – a tool for using the Web as a corpus
Word Neighbours: http://wordneighbors.ust.hk/ - for displaying how words collocate