TITLE: Are the Cram101 books any good for studying mathematics?
QUESTION [10 upvotes]: I found the book  "a Gateway to Modern Geometry: The Poincare Half-Plane" by Saul Stahl,  a bit over my budget. but browsing amazon I came across:
"Studyguide for a Gateway to Modern Geometry: The Poincare Half-Plane by Stahl, Saul, ISBN 9780763753818 Paperback "
by Cram101 Textbook Reviews
Is this studyguide any good? (the same for the studyguide for Greenbergs "Euclidean and Non-Euclidean Geometry"
And maybe the same about the other books written by Cram101 are they any good?
My interest is especially in the formulas, explaining text and proofs are a minor interest.

REPLY [2 votes]: I learned the hard way and got scammed by buying this useless study guide.
Cram101 study guides are no study guides.  Half of the book is blank.  On the non-blank pages, only one definition is provided.  The term being defined has nothing to do with the current chapter.  It’s a simple term that you already know.  On top of that, those simple terms are repeated for multiple chapters.  I don’t understand why this company is not being shut down by legal agencies.