This morning I took the CompTIA Security+ SY0-501 exam (the successor to SY-401) and passed. For what it's worth, the exam was significantly tougher than I had expected, and many questions were badly worded, some with awkward grammar where verb antecedents were hard to figure out.
I had started reading Darril Gibson's book on Security+ last year and thought it was pretty good though a bit in the weeds. That book was for the SY0-401 Exam. Since -501 was just introduced and I wanted to take the new exam (the old one is still available for awhile) I bought a copy of Emmett Dulaney's book on SY0-501.
On the whole, I would have to say that if you're serious about the exam, Gibson's book is the one. I haven't looked at the SY0-501 version, but I expect it has the same detail-oriented approach.
[Aside: The Dulaney book isn't bad, but it manages to go a little too fast over the various areas. There are bits that are presented in paragraphs that should really be in tables or more easy-to-review structures. There are also genuine mistakes. I'll add them to the online addenda for Dulaney's book, but there were a few too many blemishes in the printed version.]