Jeremy, Harmer (1998) (E-BOOK): TECH ENGLISH. Addison Wesley Longman Limited 1998. ISBN 0582-29796-6
Murphy, Raymond (2019) (E-BOOK): ENGLISH GRAMMAR IN USE. Cambridge University Press. ISBN 978-1-108-45771-2
Swan, Michael (2005) (E-BOOK): PRACTICAL ENGLISH USAGE. OXFORD UNIVERSITY PRESS. ISBN 978-0-19-442099-0