The manual you gave mewas/isvery useful. [You should know about the need for this me/you/him by now ... Focus.
The English always prefer the consistency of tenses, but you could use is to emphasize it is still useful. Use italics for examples, as I do, in order to separate from your questions.
If I use 'was' instead, do I still give the impression that it is useful? By the way, I always thought that you could leave out the subject, 'me' in this case, if you wanted it to be informal.
And also, I meant to say 'inconsistency' rather than 'consistency'. It was a typo.
>By the way, I always thought that you could leave out the subject, 'me' in this case, if you wanted it to be informal. >The manual you gave was/is very useful. That's not a subject, it's an indirect object, you gave (TO) ME. You can't leave it out in any situation and for any verb. Find the real subject. The rest I said and will not repeat.