An increasing number of HEL books have companion websites!
Jeremy J. Smith, Essentials of Early English: An Introduction to Old, Middle, and Early Modern English (2nd ed., Routledge, 2005) [suggested study plan, OE paradigms and web links of facsimiles and print books; another and more recent website has 'a set of readings of illustrative texts to accompany chapters 3-5'; also available as smartphone application]
Dennis Freeborn, From Old English to Standard English: A Course Book in Language Variation across Time (3rd ed., Palgrave Macmillan, 2006) [audio downloads, Text Commentary Book as PDF (Activities and Additional topics) and Word Book]
Charles Barber, Joan C. Beal and Philip A. Shaw, The English Language: A Historical Introduction (2nd ed., Cambridge University Press, 2009) [The tab 'Resources' shows exercises and sound files.]
Elly van Gelderen, A History of the English Language (2nd ed., John Benjamins, 2014) [chapter companion with links, glossary of linguistic terms, OED primer, example texts from each period and timeline; the companion website for the first edition (2006) is available here]
Laurel J. Brinton and Leslie K. Arnovick, The English Language: A Linguistic History (3rd ed., Oxford University Press, 2017) [Instructor Resources (passworded) and Student Resources - tutorial on using OED online, list of recommended resources, self-test exercises with answers, audio recordings of OE/ME/EModE literary texts]
William A. Kretzschmar, Jr., The Emergence and Development of English: An Introduction (Cambridge University Press, 2018) [The tab 'Resources' shows student resources.]
Dan McIntyre, History of English: A Resource Book for Students (2nd ed., Routledge, 2020) [online resources, historical events, essay questions, flashcards, texts and images, supplementary readings; the companion website for the first edition (2009) is available here]
Murray McGillivray, A Gentle Introduction to Old English (Broadview Press, 2011) ['hypertext versions of the book’s texts, interactive grammar exercises, and outlines the grammar of Old English']
Murray McGillivray, Old English Reader (Broadview Press, 2011) [texts with hyperlinked glossary]
Peter S. Baker, Introduction to Old English (3rd ed., Blackwell, 2012) ['on-line versions of all the texts in the book's anthology, exercises and a glossary’ plus OE readings on YouTube and miscellaneous resources, including the magic sheet]
Nicola Galloway and Heath Rose, Introducing Global Englishes (Routledge, 2015) [There is another webpage by Heath Rose, with 9 pages for 'Classroom Resources' – 'authentic materials related to the global spread of English'.]
Keith Johnson, The History of Early English: An Activity-Based Approach (Routledge, 2016) [chapter-by-chapter student resources, all downloadable as documents]
Stephan Gramley, The History of English: An Introduction (2nd ed., Routledge, 2019) [Chapter Resources (chapter links and introduction to OE/ME), Learning Resources (introduction to OE/ME, texts and exercises), Audio Resources (Shakespearean English and varieties of English) and Teachers' Resources]
Keith Johnson, The History of Late Modern Englishes: An Activity-Based Approach (Routledge, 2021) [Student resources are attached to the product information page and include exercises and audio files.]
Terttu Nevalainen and Elizabeth Closs Traugott (eds.), The Oxford Handbook for the History of English (Oxford University Press, 2012) [additional materials as PDF files, PowerPoint slides, weblinks and passages]
Colette Moore and Chris C. Palmer (eds.), Teaching the History of the English Language (Modern Language Association, 2019) [Complementary Materials and Additional Resources (websites, films, podcasts, workbooks, textbooks, reference books)]