As I had not grown up with Hitchhiker's Guide, I've always had a sentimental attachment to Dirk Gently since it was the first one I read in "real time", and found it as wonderfully witty and silly as Hitchhiker had ever been!
The "Horse in the Bathroom" scene is still one of my favorite passages, and as a Beatles fan, the DIALOG was simply chef's kiss!
"A Horse?" repeated Richard.
"Yes," said Reg.
Richard went out and looked up the stairs and then came back in.
"A HORSE?" he said again.
"Yes it is," said the Professor. "Wait—" he motioned to Richard, who was about to go out again and investigate— "let it be. It won’t be long."
Richard stared in disbelief. "You say there’s a horse in your bathroom, and all you can do is stand there naming Beatles songs?"
The Professor looked blankly at him.
"Listen," he said, "I'm sorry if...I alarmed you earlier, it was just a slight turn. These things happen, my dear fellow, don't upset yourself about it. Dear me, I've known odder things in my time. Many of them. Far odder. She's only a horse, for heaven's sake. I'll go and let her out later. Please don't concern yourself. Let us revive our spirits with some port."
"But...how did it get in there?"
"Well, the bathroom window's open. I expect she came in through that."
Richard looked at him, not for the first and certainly not for the last time, through eyes that were narrowed with suspicion.
"You're doing it deliberately, aren't you?!" he said.
Oh My GOSH, I remember reading this and cracking up out loud! "Yes It Is", "Wait", Let It Be", "It Won't Be Long", I did not even realize they were Beatles titles til Richard pointed it out, and when I re-read it, I was DYING! 
I remember trying to memorize the exact dialog to repeat to my friends the next day, it was so cool how Adams stung together those Beatles titles together! Just more wit out of the mind of Douglas Adams!
