Does the USA Patriot Act require probable cause before law enforcement
can ask for library records?

Hello.

No, the Patriot Act does not impose a "probable cause" requirement for
library record searches.

See:

"Section 215: Access to Records Under Foreign Intelligence Security Act (FISA)
# Allows an FBI agent to obtain a search warrant for ?any tangible
thing,? which can include books, records, papers, floppy disks, data
tapes, and computers with hard drives.
# Permits the FBI to compel production of library circulation records,
Internet use records, and registration information stored in any
medium.
# Does not require the agent to demonstrate "probable cause," the
existence of specific facts to support the belief that a crime has
been committed or that the items sought are evidence of a crime.
Instead, the agent only needs to claim that he believes that the
records he wants may be related to an ongoing investigation related to
terrorism or intelligence activities, a very low legal standard."
source: University System of Georgia
http://www.usg.edu/homelandsecurity/presentations/pa_library.phtml


"(1)   There is no probable cause requirement for a FISA search;"
source: 
Harvard Law School: INTRODUCTION TO MODULE V: THE USA PATRIOT ACT,
FOREIGN INTELLIGENCE SURVEILLANCE and CYBERSPACE PRIVACY[1]
http://cyber.law.harvard.edu/privacy/Introduction%20to%20Module%20V.htm


"IV. SECTION 215...
...Now the FBI needs only to certify to a FISA judge, with no need for
probable cause, that the search protects against terrorism.  The judge
has no authority to reject this application."
source:
Southeastern Louisiana University: Patriot Act Presentation
http://www.selu.edu/Academics/Depts/FacSen/libraries_patriot_act.html

"Questions and Answers About Section 215 of the USA PATRIOT ACT...
... This provision permits the F.B.I. to obtain orders from a judge
under certain limited circumstances. Production of tangible items,
such as records, to a court or an investigative agency at a future
date has been not considered by the courts to be a "search" covered by
the Fourth Amendment. Thus, there is no requirement to establish that
there is probable cause that the item to be produced is evidence of a
crime."
source:
US Department of Justice: Eastern District of Michigan Counter-Terrorism
http://www.usdoj.gov/usao/mie/ctu/Section_215.htm

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search strategy:
library records "probable cause" "patriot act"
"section 215" library "probable cause"

I hope this helps.

Thanks.  This looks like just what I needed.