Access to the decrypted messages of the German Enigma, called ULTRA, was very limited. Everyone working for Bletchley Park (BP) or in contact with BP had to sign a life-long secrecy act with stiff consequences for those who would break the silence.
As a rule: for ULTRA information to be used there had to be a second source to mask the fact that the Enigma had been broken. On top of that it was avoided to have tactical commanders to act upon this ULTRA INTEL because it would risk that the Germans found about it. Also not on every bit of ULTRA was acted, also because it was a lot and not all information was that clear.
We know that Montgomery had access, and all of his Army commanders, as well as their chief Intelligence Staff Officers (ISO). More specifically: did Lieutenant General Sir Brian Horrocks, commander of the XXXth British Corps have access to ULTRA messages in Operation Market Garden?
We know now - since the secrecy on ULTRA has lifted - that there was overwhelming ULTRA INTEL that the Germans were building up their forces and that Horrocks was sent into battle with insufficient gun power.
A lot of research has been done on the belated attack on the Scheldt estuary (oct-nov '44) where the neglected ULTRA messages played a decisive role on the battles that followed.
The question here relates to Operation Market Garden and the role of SIGINT. Since only in the past years the bulk of ULTRA messages have been studied it becomes clear that some histories have to be rewritten.
For some background research have a look here: at ADA406861. The master thesis has good sources.
I perused hundreds of ULTRA messages at Discovery.nationalarchives.gov.uk to corroborate findings of this thesis and of the publication of Bennet mentioned below.
And some interesting books can be read at archive.org:
Ultra in the West by Ralph Bennet. This book prints the numbers of the ULTRA messages next to the text and makes it very easy to relate it.
The Ultra Secret by F.W. Winterbotham. This book makes clear that only Army level got access to ULTRA messages. Though this book is not without controversy it is a useful source.