So I wanted an overview of Old English and that's what I got. It's literally an introduction to Old English [OE]. I should imagine that someone who wanted to learn Old English would use this as a starter-for-ten. In and of itself I can't imagine this being anything other than a springboard for learning the language but in that it's supremely useful.
Hogg is very careful and erudite and has the tone of someone who's very well-heeled as a teacher. It's got an excellent, well-structured overview of the morphology, orthography [per se], sources, syntax, grammar (&c) of OE. Hogg is also very clear on the limitations - some of which naturally arise from the limited nature of the corpus, some of which are a question of limiting the audience's exposure to the finite complexities of the language. Fiddly aspects (eg the case system, word-order) are dealt with clearly but respectful of the intricacies and apparent contradictions.
Not a 'difficult' book to read - I'm by no means a grammarian and I whizzed through - but certainly one that'd bear careful attention.
I'd be cautious to suggest this as a book to learn Old English from - it is an introduction and while it's certainly got a substantial glossary / OE > Modern English dictionary, its remit is clearly to give an overview of the language. Within that capacity, it's staying on my shelves only until someone else asks if I know any good books on Old English.