The man that appeared to Cooper's mom before she died and was climbing through the window.  I think Cooper dreamed about the Blue Man as well but I know her mom saw him.  Is she referring to the denim-clad BOB? Maybe her memory was fuzzy because it was a dream and that is all she could remember. If so, that is really cool that BOB was reaching out to Cooper before he even came to Twin Peaks.

"Harry, I'm going to let you in on a little secret. Every day, once a day, give yourself a present. Don't plan it, don't wait for it, just let it happen. Could be a new shirt at the men's store, a catnap in your office chair, or two cups of good, hot, black coffee. Like this."  -Dale Cooper

A certain place is going to freeze over when One Suave makes a "serious" reply, but hey, that is a part of his "charm".

I am not lying when I predicted while I was writing this question that I bet One Suave responds and makes a reference to the Blue Men Group, whtaver the heck they are called, and like clockwork, OSF didnt disappoint
 

"Harry, I'm going to let you in on a little secret. Every day, once a day, give yourself a present. Don't plan it, don't wait for it, just let it happen. Could be a new shirt at the men's store, a catnap in your office chair, or two cups of good, hot, black coffee. Like this."  -Dale Cooper

Perhaps it was this man:

Or BOB. Whatever. 

Logic Hat Online- logichat.org

"Harry, I'm going to let you in on a little secret. Every day, once a day, give yourself a present. Don't plan it, don't wait for it, just let it happen. Could be a new shirt at the men's store, a catnap in your office chair, or two cups of good, hot, black coffee. Like this."  -Dale Cooper

Had noticed this thread a while ago but didn't read the responses as I had not finished the book.

Now I've finished the book and was wondering '...who was the blue man?' (among other things).

Now, I'm surprised to find no theories or explanations in this thread.

So please, '...who was the blue man?' 

"Harry, I'm going to let you in on a little secret. Every day, once a day, give yourself a present. Don't plan it, don't wait for it, just let it happen. Could be a new shirt at the men's store, a catnap in your office chair, or two cups of good, hot, black coffee. Like this."  -Dale Cooper

"Harry, I'm going to let you in on a little secret. Every day, once a day, give yourself a present. Don't plan it, don't wait for it, just let it happen. Could be a new shirt at the men's store, a catnap in your office chair, or two cups of good, hot, black coffee. Like this."  -Dale Cooper

I also thought the book added some extra perspective to the TP world.

I thought that was the point.

Still like to know who the blue man is?

No offense taken. 

Most people criticize the Twin Peaks books for their inconsistencies with the series. Then again, the series and the film are not without their inconsistencies.

Personally, I think Lynch's close involvement with daughter Jen's penning the Diary, and Frost's son with Cooper's are reason enough to enter them as evidence in any Peaks discussion.  They are "official" books, like the Access Guide.  I'm always impressed by the authenticity of Cooper and Laura's 'voices' in the book.

For me those books serve pretty much as nostalgic trips to my teenage years only. There still are some nice stuff in them, but hey, let's face it; those books are not exactly masterpieces.

And oh, I have no idea about the blue man.