Care to say why? I've coincidentally just been referring to this guide whilst I explore some CLOS. It seemed adequate to me. (Not saying gigamonkeys isn't great - it is just a bit more long winded..)
I found this: pg's Ansi Common Lisp while useful will teach you only those parts that pg believes are good in CL. OTOH, PCL teaches you the language almost completely, leaving aside the author's preferences if any.
This may also be one of the reasons why the avoid advice! But ACL is also useful. In fact I started off with ACL and having come from C++, Java suffering from OO overdose, completely skipped anything OO. Was a whiff of fresh air learning that way.
Only when I turned to PCL and had been treated of bad OO by the earlier exercise, did I realize that CLOS is not the typical OO. In fact at times i think it is just no OO, but cannot articulate why for now.
Pair up PCL with LOL (let over lambda) and it can be fun.
I just finished reading Sonya Keene's Object-Oriented Programming in Common Lisp cover-to-cover. There is a lot of stuff in there that just can't be covered by a brief guide for sure, a lot of pretty useful stuff, knowledge of which would quite possibly cause one to structure one's program in a substantially different way, if one knew about it. I highly recommend reading it if you intend to use CLOS for anything serious, and poring over the extended case study implementing streams is useful too, tedious though it might be. There are, however, some things that got changed between the printing of the book and the finalization of CLOS as we see in CL today. We write a describe-object method instead of a describe method, for example, and there is no longer a with-added-methods special form.
PCL is much better. Keene too. Seek out Joe Marshall's Warp Speed intro too.