How do I know what printing of a book I have?

That is not always correct. Some publishers start the number line at 2, with 1 being the first edition.

Also, if your number line started at 3 4 5, you would probably have a first edition, 3rd printing unless the book was published by one of the non-conforming presses.

And, even if your book says first edition and has a number line starting with one, if the DJ does not have a price or there is a stamp on the rear cover, you may have a book club edition rather than a first. To further confuse matters, some book club editions are true firsts.

And finally, if your book is from a small press, there may be no number line or reference to edition or printing as it may have been a one printing run.

Here are some references for you:

http://www.travelinlibrarian.info/writing/editions/

http://www.litterascripta.com/bibliomania/ident.shtml

There are entire books written about how to differentiate between first editions and subsequent printings. Your readers should be careful and do their research before they start buying and or trying to sell true firsts.

Thanks for the additional information, Patricia. It is, as you say, very important to differentiate between printings and editions. A subsequent printing is still the same edition, and theoretically nothing has changed of note. A subsequent edition should reflect new or updated editorial content.