Reddit mentions of An Introduction to Old English (Edinburgh Textbooks on the English Language)

It’s a great little book, Mark Atherton is a brilliant scholar of Old English (and his work on Tolkien is equally good, if you’re a fan). It starts very easy, then works up through real texts and cultural context. The same is true of Hough & Corbett’s Beginning Old English– lots of original sections of prose and poetry, starting with basics and building enjoyment alongside the language.

If you want something that’s more academic from the start, I’d recommend Peter Baker’s Introduction, which contains a good grounding in grammar more generally, or the more linguistics-based one by the late Richard Hogg. The best overall textbook is undeniably Mitchell & Robinson’s Guide – but it is dense and (despite what the authors claim) not easy to use in independent study. Their sections on syntax are vast and comprehensive, however, so that might be what you’re looking for.