Just returned from Canada vacation and I wanted my passport stamped . , but I did not know that I could have asked the agent to do so until we had passed the border back into US. Is it too late to go to passport Office (or someplace in Maryland) and get a stamp in my book?
You might try at one of CBP's deferred inspection sites. More information is available at their page on the subject. There is one such site in Maryland, at BWI airport.
If you are a US citizen, however, you are probably out of luck, because not stamping your passport is not an error, and, as the page notes
The Deferred Inspection Sites will only correct errors made at the time of entry.
If you are not a US citizen, you will probably be given an entry stamp with the proper date, but you will probably need some sort of evidence to support your assertion that you entered the US on that date.