You could punctuate this in a few ways, in decreasing levels of formality

With a colon (:)

I forgot to ask: did you find that book I told you about?

With a dash (–)

I forgot to ask—did you find that book I told you about?

With an ellipsis (…)

I forgot to ask…did you find that book I told you about?

The colon is the most formal, the ellipsis quite informal. I personally would use a dash. The ellipsis here is used to indicate a pause or an unfinished thought, which some purists might object to (claiming that an ellipsis can only be used to indicate something which has been left out).

You could punctuate this in a few ways, in decreasing levels of formality

With a colon (:)

I forgot to ask: did you find that book I told you about?

With a dash (–)

I forgot to ask—did you find that book I told you about?

With an ellipsis (…)

I forgot to ask…did you find that book I told you about?

The colon is the most formal, the ellipsis quite informal. I personally would use a dash. The ellipsis here is used to indicate a pause or an unfinished thought, which some purists might object to (claiming that an ellipsis can only be used to indicate something which has been left out).