Let me quote from a recent response I posted on this topic in the Magic Cafe & then I'll add an addendum:
"Just a brief note on used book prices. In my experience of 10 years as a magic bookseller, prices are based primarily on 3 variants: rarity, edition & condition.
By rarity, I mean how easy is it to get the book. E.g., how many copies have been offered in the previous year (or 10 years)? Is a reprint available? Etc. Often, a "rare" book is reprinted and the original loses some of its value, except to collectors. A perfect example of this is Kaufman's reprint of "Greater Magic," which, IMO, is superior in several regards to the original. However, this too is out of print & now "collectible."
By Edition, I'm talking about whether a book is a first edition, later edition, reprint, etc. I can go on & on about this, but some editions (generally firsts) are just more valuable than others. We pride ourselves on being virtually 100% accurate in describing editions (altho I admit to the occasional mistake).
By Condition, I mean what shape the book is in. How accurate is the description? Does the book have a dust jacket? Etc. Do you care? Again, we take great effort in describing the condition of our books.
Many people say that they're just interested in the information, not the condition, edition, etc. Fine. We sell many great magic books for $3 or less. There are cheap copies available of many magic books, although one should factor in the value of one's time in searching for them. Moreover, I find it a bit ironic that people who claim to just be interested in the "information" feel differently about other purchases. E.g., do you care if clothes are used, tattered or new as long as they cover your body? "
Two additional points:
1) My "Price Guide to Magic Books," while a bit out of date, can give a good guide to the relative value of books, as well as some help in identifying editions. We have 3 copies left.
2) Regarding "13 Steps to Mentalism," this is a great book that has gone through many editions & printings. I believe the book is still in print for $25, altho we are out of stock. However, the first hardcover edition in a dust jacket (different from the new design & published by Corrinda himself) has fetched upward of $200 dollars. The individual "steps" are often available as well, published by Corrinda as pamphlets.
Hope this helps.